### sevlll777's blog

By sevlll777, history, 4 months ago,

Big greetings, Codeforces!

I am happy to invite you to Codeforces Round 908 (Div. 1), Codeforces Round 908 (Div. 2), which will be held on Nov/07/2023 17:35 (Moscow time).

This round will be rated for everyone. In both divisions, you will be given 5 problems and 120 minutes to solve them. All problems were cooked by me (sevlll777).

The traditional thanks-list to everyone who took part in the creation of the round. Thanks,

I hope you will like the problemset and ideas hidden in the problems! It's guaranteed that statements are understandable, short, and, of course, ✨ stylish ✨.

Have fun!

Score Distribution:

Div. 1: $500$ — $1000$ — $1250$ — $2250$ — $2750$

Div. 2: $500$ — $750$ — $1500$ — $2000$ — $2250$

UPD: Editorial

UPD2: Congrats to the chAAAmpions!

Div.1:

Div.2:

• +175

By sevlll777, history, 4 months ago,

I'm very very sorry to all Div2 participants for unclearness in statement of A, and not including notes in the statement of B, hope it didnt ruined a contest for you. Thank you all for participating, I hope you enjoyed non-empty subset of the problems! You can rate the problems of the round in the corresponding spoilers.

Hint 1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 1.1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Tutorial
Solution
Rate the problem
Hint 1
Hint 1.1
Hint 1.2
Hint 2
Tutorial
Solution
Rate the problem
Hints: Natural Way
Hints: Believers Way
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Hint 4
Hint 5
Hint 6
Tutorial
Solution
Rate the problem
• +26

By sevlll777, history, 6 months ago,

Thanks for joining the contest!

Tutorial
Solution
Tutorial
Solution
Tutorial
Solution
Tutorial
Solution
Tutorial
Solution
Tutorial
Solution
Tutorial
Solution
• +82

By sevlll777, history, 6 months ago, translation,

Hello Codeforces! Codeforces Round 895 (Div. 3) will start at Sep/07/2023 17:35 (Moscow time). You will be offered 7 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have a rating of 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ICPC). Thus, solutions will be judged on preliminary tests during the round, and after the round, it will be a 12-hour phase of open hacks.

You will be given 7 problems and 2 hours and 15 minutes to solve them.

Note that the penalty for wrong submission in this round is 10 minutes.

Remember, that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participant of the third division, you must:

• take part in at least five rated rounds (and solve at least one problem in each of them)
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Problems have been created and prepared by: Alexdat2000, FairyWinx, sevlll777, Vladosiya, и MikeMirzayanov.

We would also like to thank:

1. Vladosiya for the amazing coordination, and help with preparing and balancing a problemset.
2. MikeMirzayanov for Polygon and Codeforces platforms.
3. Ormlis for black-red testing.
4. zwezdinv, BledDest for red testing.
5. Sokol080808, diskoteka, Sweezy, vladmart, Kniaz, Tima, Riblji_Keksic, 74TrAkToR, pavlekn for yellow testing.
6. moonpie24, gs20036, Kolychestiy for purple testing.
7. martin0327, tnaito, no_mind, Pa_sha, SashaT9, ctraxxd, ezdp, BF_OF_Priety for blue testing.
8. NerfThis, _SADIEM_, YudoTLE, sayed_4 for cyan testing.

Good luck!

UPD: Editorial

• +288

By sevlll777, 11 months ago, translation,

Hello Codeforces!

I am happy to invite you to Codeforces Round 860 (Div. 2), which will be held on Mar/26/2023 17:35 (Moscow time).

This round will be rated for participants with rating lower than 2100. Participants with a higher rating are invited to participate in the round unofficially.

You will be given 6 problems and 120 minutes to solve them. All problems were authored and prepared by me.

The traditional thanks-list to everyone who took part in the creation of the round:

🤴 DishonoredRighteous for coordinating the round

🐞 gyh20 for black-red testing of the round

😈 feecIe6418, iakovlev.zakhar, Dart-Xeyter, Adam_GS, ShuiLaoshi, golikovnik, Gary2005 for red testing of the round

🐫 NemanjaSo2005, Alexdat2000, Kon567889, tem_shett for orange testing of the round

👾 SlavicG, Psychotic_D for purple testing of the round

🐳 C2A, Masha237, ayhan23, Khonshu08, Brahma_tet for blue testing of the round

👽 Lord_David for green testing of the round

🦄 mejiamejia for help with testers for the round

🤡 sevlll777 for the problem, without which the round would be unbalanced, and the problems that were not included in the final problemset

🎅 MikeMirzayanov for the amazing Codeforces and Polygon platforms
ㅤㅤㅤㅤ

Personal recommendations

I sincerely hope that you will find the problems interesting and you will enjoy solving them. Good luck!

Score Distribution:

500 — 750 — 1250 — 1750 — 2250 — 3000

UPD: Editorial

UPD2: Congrats CHAMPIONS!

Unofficially:

Officially:

First AC:

A: nifek

D: aryan12

• +822

By sevlll777, history, 11 months ago, translation,

Thank you all for participating, I hope you enjoyed the problems! You can rate the problems of the round in the corresponding spoilers.

Hint 1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 1.1
Hint 1.2
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 1.1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 2.1
Hint 3
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Hint 3.1
Hint 4
Tutorial
Solution
Rate the problem
• +262

By sevlll777, history, 2 years ago, translation,

Thank you for participating, we hope you enjoyed the problems! We kindly ask you to rate each of the round's problems in the corresponding spoiler in order to improve the quality of future contests.

You can also check video editorials of problems B and C on ak2006 Youtube channel.

All problems were prepared by Alexdat2000 with the help of coauthors.

Why didn't AI participate

Hint 1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Tutorial
Solution
Rate the problem
Hint 1
Hint 1.1
Hint 2
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 2.1
Hint 3
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Hint 4
Tutorial
Solution
Rate the problem
Hint 1
Hint 2
Hint 3
Tutorial
Solution
Rate the problem
• +333

By sevlll777, history, 3 years ago,

Hi!

Today I was surfing Wikipedia and came across this article — Palindromic prime

This article says that the largest known palindromic prime is $10^{474500}$ + $999 * 10^{237249} + 1$.

Well it is easy to see that this number is palindrome, but... why is it prime?

I don't find any proof, and I am really curios — how to proof that this number is prime, when number is quite big?

• +79

By sevlll777, history, 4 years ago,

DISCLAIMER: sorry for my poor english, hope you can understand this text :)

Hello!

Firstly: Codeforces is a beautiful platform, sure.

In my mind hacks became useless. Why? Ok, see:

Many easy problems (D1AB/D2ABCD) are "multitest" problems. It is really hard to hack them, because pretests are very strong. But what if I want to hack some hard problems? Hm, i tried to hack some hard problems, but in div2 rooms are very small, and i discovered that there were only 2-3 people who solved some hard problems.

Anyway, i saw that jqdai0815 in one of his screencasts was very annoyed about it too.

But if i will try to hack D2B for example, i would find roughly 10 people, and the chance of succes hack is very small, so it is useless to spend time on it.

So, it is really hard to hack someone, and we can see that the quantity of hacks is very small in some previous rounds!

-

What is my solution? Ok, lets make rooms bigger from 40 to 80.