Errichto's blog

Codeforces Round #356
By Errichto, 9 years ago, In English

Hello Codeforces.

The CF Round #356 will happen on 8-th June (exact time). You will get five problems to solve in two hours. The round is rated.

I encourage you to read other problems if you don't like or can't solve something. The scoring distribution will be announced.

MikeMirzayanov and GlebsHP make the round possible. Also, thanks for Radewoosh, kostka, johnasselta, AlexFetisov and (more to be added?) for their amazing help. And I want to thank my girlfriend because there would be no Limak without her.

It's my first standard round. Still, you should get nice interesting problems. You will meet Limak, who is usually a little polar bear. Here he is, preparing one of problems.

I wish you great fun and no frustrating bugs.

EDIT — It's recommended for both divisions to read the Interactive Problems Guide before the round.

EDIT2, SCORING

div2: 500-1000-1750-2250-2750
div1: 750-1250-1500-2000-2500

EDIT3

The editorial with codes is ready.

WINNERS, congratulations!

  1. Petr
  2. ainta
  3. halyavin
  4. jcvb
  5. brandnewnode

and Division 2:

  1. Y_UME
  2. kaq
  3. BehtarinFake
  4. Zoli9
  5. Jeannette

Thank you all for participation and see you next time. And regards from Limak, a bear.

  • Vote: I like it
  • +687
  • Vote: I do not like it

Comments Comments (174)
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9 years ago, # |
  Vote: I like it +106 Vote: I do not like it

Looking forward to it! Such a long time since the last Div 1 round.

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    9 years ago, # ^ |
      Vote: I like it -207 Vote: I do not like it

    I am taking part in the College Entrance Examination in china.

    But there is a conflict between the Examination and this contest.

    But nothing is more important than become a purple boy.

    So I give up the College Entrance Examination.

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      9 years ago, # ^ |
        Vote: I like it +86 Vote: I do not like it

      I have seen your big font several times, and I think it isn't beautiful...

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        9 years ago, # ^ |
          Vote: I like it -82 Vote: I do not like it

        WOW

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        9 years ago, # ^ |
          Vote: I like it -9 Vote: I do not like it

        He's just very afraid of gaokao (高考) )))

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          9 years ago, # ^ |
            Vote: I like it +3 Vote: I do not like it

          I apologize to all the English-speaking, below I will explain for Russian — that such GAOKAO

          Поясню для тех, кто не в теме, китайский ГАОКАО (高考) — это аналог российского ЕГЭ, только намного круче и ответственнее. И он проходит единым экзаменом в один день по всей программе, а не как в России разбит на предметы. Китайские школьники сдают этот экзамен на стадионах, гигантских актовых залах, часто не в свое городе, чтобы не было возможности "договориться". По результатам набранных экзаменов идет распределение во вузам, техникумам, училищам. Вроде тоже самое, что и у нас в России, НО — всего 5-7% выпускников китайских школ имеют возможность продолжить учебу в учебных заведениях. Остальные — работать на фабрики, поля и улицы! Поэтому истерика при сдаче просто сумасшедшая. При этом корочки ВУЗа дают высокий шанс на успешное будущее. На фото — проводы выпускников на экзамен

          Вот поэтому и я понимаю, почему КРУПНОПИШУЩИЙ нервничает ))

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      9 years ago, # ^ |
        Vote: I like it +16 Vote: I do not like it

      Retarded

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      9 years ago, # ^ |
        Vote: I like it +2 Vote: I do not like it

      why you can comment on this blog during the math exam...

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        9 years ago, # ^ |
          Vote: I like it -22 Vote: I do not like it

        Math is too simple for me. I have solved all the problem quickly, and hand in my paper ahead of schedule.

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          9 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Indeed the JIBACANYANG,Orz

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9 years ago, # |
  Vote: I like it -66 Vote: I do not like it

You are a programmer AND you have a girlfriend!!!

you are my role model dude.

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9 years ago, # |
  Vote: I like it +745 Vote: I do not like it

"And I want to thank my girlfriend because there would be no Limak without her." congratulations on becoming a father!

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    9 years ago, # ^ |
    ← Rev. 3   Vote: I like it +364 Vote: I do not like it

    Your comment is unbearable.

    EDIT: And I will kill you for this comment. I'm not going to help you with problems anymore. -_-

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    9 years ago, # ^ |
    ← Rev. 2   Vote: I like it +37 Vote: I do not like it

    so his girlfriend is a polar bear?

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    9 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    Why don't you thank that beautiful bear in your arms!?
    Btw I congratulate you in-advance :D

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    9 years ago, # ^ |
      Vote: I like it +59 Vote: I do not like it

    If you look closely you can see that Swistakk is also holding a little bear , i guess polar bear is the most wannabe female in Poland.

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    9 years ago, # ^ |
      Vote: I like it +84 Vote: I do not like it

    Now I'm even curious what will grow larger: the rating of your comment or the rating of this blog post.

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      9 years ago, # ^ |
        Vote: I like it +57 Vote: I do not like it

      I and Errichto have closely observed that stout fight for two days xD.

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        9 years ago, # ^ |
          Vote: I like it +6 Vote: I do not like it

        I am also very curious if there is any relationship between the bear on your profile photo and Limak? :))

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          9 years ago, # ^ |
            Vote: I like it +3 Vote: I do not like it

          No. That photo was taken back in 2011, way before Limak which I don't know how arose.

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    9 years ago, # ^ |
      Vote: I like it +31 Vote: I do not like it

    More votes on your comment than Errichto's blogpost . Looks like you are distributing your chocolates to downvote his blogpost and upvote your comment. :P

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9 years ago, # |
  Vote: I like it +5 Vote: I do not like it

You are my favourite problem setter! Too bad I will miss this round.

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9 years ago, # |
← Rev. 3   Vote: I like it +38 Vote: I do not like it

Looking forward to "Bear and Forgotten Tree Girlfriend"! :)

Anyway, this contest will surely be great as always. Thank you Errichto!

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9 years ago, # |
  Vote: I like it +2 Vote: I do not like it

Yes! The stories with Limak are always nice to read :)

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9 years ago, # |
  Vote: I like it +63 Vote: I do not like it

Errichto be like, Petr's getting closer to me on the contribution list, lemme prepare a new round and make his mission impossible lol .
You always come with great problems, eagerly waiting for the round and hope I'll be able to participate, thank you .

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    9 years ago, # ^ |
      Vote: I like it +51 Vote: I do not like it

    You don't seem to know how big difference it is (currently between 162 and 175). Function contribution(upvotes on blog, upvotes on comments) is rather like a cubic root or logarithm rather than linear :P.

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9 years ago, # |
  Vote: I like it +13 Vote: I do not like it

I'm looking forward to it. You wrote some very nice problems in the past. I hope there will be many algorithmic problems :D

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9 years ago, # |
  Vote: I like it -18 Vote: I do not like it

Limak has a great subliminal message. Reverse it first: Kamil. It is the name of first Turkish polarbear manufacturer.

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    9 years ago, # ^ |
      Vote: I like it +33 Vote: I do not like it

    And its the name of the person who is preparing this round as well :)

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    9 years ago, # ^ |
      Vote: I like it -12 Vote: I do not like it

    Sen hala burdamisin la?

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9 years ago, # |
  Vote: I like it +10 Vote: I do not like it

my 1st div1 participation , am I gonna go back to div2 ?? I hope not :p

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    9 years ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    anyway , I like blue more than pink :v :v and it was a nice try

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      9 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      You were purple before, not pink.

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9 years ago, # |
  Vote: I like it +6 Vote: I do not like it

Errichto, Your Problem Statements are So clear to me. I Solved Your problems in Hackerrank too. Thanks You.

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9 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Good Luck & Have Fun!

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9 years ago, # |
  Vote: I like it +128 Vote: I do not like it

We get it, you have a girlfriend.

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9 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Tbh, Limak scares me. The later problems turn out to be DPs, which are scary for me. The quality of questions are great, though.

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9 years ago, # |
  Vote: I like it +4 Vote: I do not like it

That's GOOD

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9 years ago, # |
  Vote: I like it +22 Vote: I do not like it

Guess there will be statement like that:

As Limak is a bear and he can't solve the problem by himself, so he ask you for help. Can you help him?

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    i think it's a Plan to Strengthen relations Between Polar Bears and Humans

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      9 years ago, # ^ |
        Vote: I like it -8 Vote: I do not like it

      or maybe the polar bears are some kind of alien and Errichto is one of their top spies sent to invade the earth he has already attacked Swistakk (notice the polar bear in his hands.) soon the polar bears might invade the earth. that would be unbearable

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9 years ago, # |
  Vote: I like it +21 Vote: I do not like it

This may be strange, but I'm becoming a fan of Errichto, his posts, comments and problems. :)

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9 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Wish me good luck guys I really feel like I'm gonna make it!!

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    9 years ago, # ^ |
      Vote: I like it +79 Vote: I do not like it

    What do you do... 50 push-ups for every Accepted ?

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9 years ago, # |
  Vote: I like it +10 Vote: I do not like it

I'm wondering if this little polar bear is preparing a problem about powers of prime numbers ? :p

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    let's hope not

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    9 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    Wow, you guessed it right. Powers problem and primes problem.

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      9 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      and he solved none of them

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        9 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        ya i got good prediction :D , but bad rating :( ... but good prediction :D !!!

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9 years ago, # |
  Vote: I like it +5 Vote: I do not like it

i really hope that the pretests would be strong enough , because it's sucks when you spend a lot of time working on Problems and gets the same position of someone spent all the time refresh room page to hacks someone else solutions.

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    9 years ago, # ^ |
      Vote: I like it +33 Vote: I do not like it

    Hey, hacking is hard work! It's much harder to get 500 or 1000pts by hacking than solving a problem. Besides, once you get the hang of it it's super fun. Also, it's good to learn to code without relying on strong outside tests if you're ever gonna do coding outside of competitions.

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      9 years ago, # ^ |
      ← Rev. 3   Vote: I like it -6 Vote: I do not like it

      [Deleted]

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        9 years ago, # ^ |
          Vote: I like it +54 Vote: I do not like it

        Do you prefer it to fail on final tests?

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          9 years ago, # ^ |
            Vote: I like it +52 Vote: I do not like it

          It's locked, at least he can live as happy for more 15-20 minutes :P

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          9 years ago, # ^ |
            Vote: I like it +4 Vote: I do not like it

          Errichto You are merciful and you'll don't set tricky test case as a final test ...-_-... and maybe not :P

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          9 years ago, # ^ |
            Vote: I like it +13 Vote: I do not like it

          I suppose we all would rather to be hacked way before the round ends, so we could fix it unless you have already locked the problem (which is a lost case anyway).

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            9 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            exactly , I think hacking is a good thing

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hope I can get my first 4/5 at your round, after 4 unsuccessful try -_-

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9 years ago, # |
  Vote: I like it -20 Vote: I do not like it

What do you mean by "your first standard round" — were the others prepared by you a non standard?

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    9 years ago, # ^ |
      Vote: I like it +17 Vote: I do not like it

    He has prepared Educational Rounds, VK Cup, India Hacks, Thanks-Round. Go to Problemsetting tab. So this is his first Standard Round.

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

It's fun that the last CF Round I took part in is on the Chinese Spring Festival, and this one is on the Dragon Boat Festival.

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9 years ago, # |
  Vote: I like it +16 Vote: I do not like it

Thumbs up for those playing cards from World Finals 2012! :)

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9 years ago, # |
  Vote: I like it +28 Vote: I do not like it

It's recommended to read the Interactive Problems Guide prepared by Mike Mirzayanov.

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9 years ago, # |
  Vote: I like it +8 Vote: I do not like it

i think the second problem will be interactive as second card is hidden in the image .

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    9 years ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    The interactive problem will be both for Div. 1 and Div. 2, according to the main post. So, it cannot be the Div. 2's second problem.

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9 years ago, # |
← Rev. 2   Vote: I like it +58 Vote: I do not like it

So it is not a standart round as well :p

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9 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Having interactive problems adds to fun of Competitive Programming. :)

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

It's about time I left the green phase without a second comeback

Good luck everyone

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Good luck, I hope that too for myself !

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9 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Oh, div2 C scores 1750, I have a bad feeling..

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    9 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    C is easy just like typical rounds. But D and E, too difficult for me. the 5th unsuccessful attempt to get a 4/5.

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9 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Feels like experimental round, poor you Errichto

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9 years ago, # |
← Rev. 2   Vote: I like it +17 Vote: I do not like it

I had great fun solving Div2/C. Thanks for the nice problems, Errichto!

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9 years ago, # |
← Rev. 5   Vote: I like it -16 Vote: I do not like it

My solution to problem C.Whenever i tried to submit it , my browser get stuck on the same page. More interesting part is that submitting this solution to any other problem results the same.

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9 years ago, # |
  Vote: I like it -38 Vote: I do not like it

Why am I getting WA for Div2 C?

int sieve[1000];

set st;

bool isPrime(int num) {

return (st.find(num) != st.end());

}

int main() {

//t(); 

st= { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 }; 


vi v; 

for (int i = 2; i <= 10; i++) {

    cout << i << endl;

    fflush(stdout);

    string s;
    cin >> s; 

    if (s == "yes") v.pb(i); 

}

if (v.size() == 0) { cout << "prime" << endl; fflush(stdout); return 0;  }

if (v.size() ==  1) {

    if (!isPrime(v[0])) { cout << "composite" << endl; fflush(stdout); return 0; }

    else {

       cout << v[0] * v[0] << endl;
       fflush(stdout);
       string s; 
       cin >> s; 

       if (s == "yes") { cout << "composite" << endl; fflush(stdout); return  0; }

       else { cout << "prime" << endl; fflush(stdout); return 0;  }

    }


}


if (v.size() > 1) { cout << "composite" << endl; fflush(stdout); return 0; }



return  0;

}

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Consider number 74 = 2 * 37 for example.

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

i got stuck on C because i didn't take perfect squares in account

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9 years ago, # |
  Vote: I like it +7 Vote: I do not like it

How to solve Div 2 — D?

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    9 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    What I coded up and finished a bit after the end was this:

    First, generate first few terms of sequence, and oeis the rest: http://oeis.org/A055402

    Then hardcode those values, and for a given m pick the best value you can. Call this best value v. Decompose v into its cubes, then(from highest to lowest) try incrementing each cube(n^3 becomes (n+1)^3) while making sure the sum doesn't exceed the given m. Then output this sum. I didn't get to submit, so I'm not sure if this is completely correct

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      9 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Oh, I knew I weren't the only one who tried to look up this very sequence in OEIS.

      Came up with almost the same approach and got WA 4 (see 18325136).

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9 years ago, # |
  Vote: I like it -6 Vote: I do not like it

According to me , the 3rd problem should be able to differentiate between coders and the number of people who solve should be <=700,800. That was definitely not the case today.

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9 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

image 2 of my submissions were submitted at the exact same second . Can one of these be removed while considering penalty >.<

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Problem Div2 C was really easy for hacks, because it had week pretests! For example, I hacked two people using "94" as a hack because they didn't check for 47. Another hack was "49", so no perfect squares.

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9 years ago, # |
  Vote: I like it +1 Vote: I do not like it

TNX for your perfect contest and fast editorial guys.

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9 years ago, # |
← Rev. 3   Vote: I like it +73 Vote: I do not like it

After try spending 1 hour and 45 minutes on Div1-B:

I'll need to train harder next time...

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Dont Worry the number of the contests coming are more than those which have passed :) All of us needs to train harder :)

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    9 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    I couldn't think of a legit solution either...

    I ended up using a really sketchy solution in which I check the 500000 numbers below m and take the optimal value (can't prove it's correct, let's see if I AC).

    edit: darn got WA

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      9 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      Did the same thing, WA on test 20 :|

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    9 years ago, # ^ |
      Vote: I like it +29 Vote: I do not like it

    Why didn't you try div1-C then?

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    9 years ago, # ^ |
      Vote: I like it +25 Vote: I do not like it

    Spent whole contest trying to solve B and then solved C in 20 minutes after the contest....

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9 years ago, # |
  Vote: I like it +15 Vote: I do not like it

Errichto when can we expect system testing? In 5 minutes or in a hour?

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I want to know why my solution got run time error for div2 b. can anyone pls take a look? link to submission Thanks

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I completely forgot to input m as a long long for problem D.

At least my solution probably had a bug somewhere else!

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9 years ago, # |
  Vote: I like it +22 Vote: I do not like it

Again, A massive gap between Div.2 C and Div.2 D ...

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I managed to find number of cubes in D and minimal volume. Was it the right way or this information is useless?

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9 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Forgot «47» in C... short div[] = {2, 3, 4, 5, 7, 9, 11, 13, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49} Frustraiting...

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Can anyone please explain how to obtain the 0.916666666667 in the 3rd sample test of problem D?

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    9 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    If I remember correctly, you should twice use BCD in city 1. If you get distance 0 then you win immediately. Otherwise, for sure Limak moved between cities 2 and 3 — guess one of them.

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9 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

hacked two solutions in div2 C and then realized that i was screwed ! 25 and 49 will hunt me down tonight . :(

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9 years ago, # |
  Vote: I like it -79 Vote: I do not like it

Now-a-days contests have become extremely rare. And then to provide a contests like today is so disappointing...To do so much upsolving preparing for contests..for what? An interactive problem that almost 70% of the candidates can solve? Noooo...This is just not done..

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    9 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Speed matters too you know !

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      9 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Absolutely....I wasted too much time on C and got stuck on 1200 position...if i would have solved it faster...my position would have been under 500 as I had a lot time for C :(

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        9 years ago, # ^ |
          Vote: I like it +13 Vote: I do not like it

        You could also use hacks on Div2 C even if you had lost some time, there were many participants with wrong code because the pretests were weak. After submitting my first 3 problems I ended up being somewhere near place 250, but with 3 hacks I got to the place 47 and thus I am Div1 now :)

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    If you would've solved problem D, you wouldn't be complaining. Keep upsolving, and you will keep do better and better :)

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9 years ago, # |
  Vote: I like it +46 Vote: I do not like it

Thank you for a wonderful round!)

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9 years ago, # |
  Vote: I like it +19 Vote: I do not like it

Thank you very much, Errichto! I loved the problems!

I came up with the efficient approach for C right after the contest ended and it took me some 10-15 minutes more to code it, but I enjoyed the contest nevertheless :D

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9 years ago, # |
← Rev. 3   Vote: I like it +30 Vote: I do not like it

my eyes almost got cancer from debugging this.. only realizing my l and j is swapped after the contest is finish..

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9 years ago, # |
  Vote: I like it +7 Vote: I do not like it

The waiting for rating change has just begun.

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9 years ago, # |
  Vote: I like it +24 Vote: I do not like it

Thank you for participation and for kind words. Also thanks for bad words (as long as it's something constructive/reasonable). Btw. div2 participants can now check again a picture from the blog. Do you know why is there one card hidden?

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    9 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Congratz for the contest, it was really good in my opinion (maybe difficulty raise was a little too high between div2.C and div2.D, but still I really enjoyed this contest).

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    9 years ago, # ^ |
      Vote: I like it +27 Vote: I do not like it

    Because you hided it??

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    9 years ago, # ^ |
    ← Rev. 3   Vote: I like it +9 Vote: I do not like it

    Because

    Spoiler
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9 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Awesome contest!!! :D Solved 3. Coded brute force dp for 4th one but failed to find a tiny bug in time which was resulting in wrong answers for the tescases :/

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9 years ago, # |
  Vote: I like it -32 Vote: I do not like it

My view of the points system this round 500 — 750 — 1250 — 2500 — 3500

Honestly, I miss the dynamic scoring.

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    9 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    How would it affect your position?

    (but yeah, I see that the difference between C and D was too big)

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

18329338 -_- It gives a proper output in CodeBlocks but wrong in CodeForces !!! But why ?

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

a moment of silence :D

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9 years ago, # |
  Vote: I like it +14 Vote: I do not like it

This contest was outstanding.

It's the first time I solve C during the contest and I turned green :)

thank you Errichto.

hope to see you make new contests soon. :)

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Awesome round, especially Div2/C. Looking forward to seeing more interactive problems in the future! :)

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9 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Very interesting round! The most interesting round in a while for me. Thanks for the interesting problems! Wish I could've solved div 1 C :/

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9 years ago, # |
  Vote: I like it +18 Vote: I do not like it

I think Div 1 C/D may be easier than Div B :)

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