Hello, Codeforces.

Unfortunately, we have to declare the round unrated.

Absolutely, these are not issues of the side of Codeforces code, but rather it is a hardware issue. Probably, electricity, power unit or something else. This led to unexpected repeated reboots of the server with the database. I hope that we will be able to quickly deal with this.

We apologize to the writers of the round and its participants.

P.S. Please, do not downvote the round announcement: the writers are not to blame for what happened. And in general, this time it is difficult to blame someone. Sometimes hardware fails.

Mike.

But why not!

Many people here seem to know me. Or maybe not quite? I am ready to answer your questions. Ask me anything! I plan to spend no more than 3 hours on answers. Sorry if I can't answer all the questions.

Let's more or less follow the rules from here https://www.reddit.com/r/IAmA/wiki/index

Below is actually a quote from the rules.

The interviewee begins the process by starting a post, describing who they are and what they do. Then commenters from across the internet leave questions and can vote on other questions according to which they would like to see answered.

The interviewee can go through and reply to the questions they find interesting and easily see those questions the internet is dying to have the answer to.

Because the internet is asking the questions, they're going to be a mix of serious and lighthearted, and you'll find yourself sharing all kinds of things you won't find in a normal interview.

Comments will be removed under a few circumstances:

• Abusive or harassing comments.
• Requests for personal favors from the OP (For example, "OP, can you send me a signed autograph").
• Top-level comments must ask a question.
• Comments where there would be no possibility of a real answer, especially where it is deliberately creepy or offensive.
• "I bet OP won't answer this"-type responses, which usually come after the OP has finished responding to questions.
• "Fluff," non-contributing responses from users, responding to all of the OP's comments for karma/attention.
• Repeatedly asking the same question.

Please, don't attempt to bypass the rules by adding a ? to a nonquestion.

Questions must be directed toward the individual(s) doing the IAMA.

Under our policies, astroturfing is the practice of an individual or group of individuals who plant questions in an IAmA post for a particular purpose. This kind of behavior is forbidden.

UPD 1: Oh, I'm kind of tired of doing this. Please keep asking questions. Tomorrow I will answer again. Don't forget to vote for the most interesting ones. I will not have time to answer everything, I will choose something. Thanks!

UPD 2:

Hi.

Here is the list of top-voted blog posts in 2021. Find something interesting for yourself?

Hey!

Are you celebrating the New Year already? I wish you a Happy New Year. I wish you progress, joy and health.

We decided to show the current progress in the catalog of significant Codeforces posts.

So far, quite a bit has been implemented, but somehow you can already use it! Would you like to see it? Follow the link:

As planned, the catalog will collect and classify interesting and useful posts from Codeforces. Indeed, many useful articles have been written in Codeforces over 12 years (God, already 12 years!).

The catalog will be a kind of Codeforces golden fund in the future.

The catalog is a tree of topics to which posts can be added. We will still work on its functionality, for now there are only basic features. Don't be discouraged if something is not supported, which is very necessary — just let me know!

Any red user can enable directory editing. Remember that it is extremely important to make all changes thoughtfully and carefully. The tree should not become infinite in size. You need to have a sense of proportion when detailing topics. Please add only good, useful articles to the directory. If there is a disagreement on the contents of the catalog, the administration will make the final decision.

Hope this will be helpful. And together we can put together a catalog and keep the catalog up to date.

P.S. Many thanks to KAN who helped a lot with suggestions and catalog initial content.

Hello.

Here's a small improvement for posts and comments. Likes can now be specified in their markup. Maybe this will be useful for organizing some kind of polls or collecting feedback.

Here are some examples:

Yes, I know that this feature can be improved. I would prefer that we try to use it a little and understand what exactly is missing the most. Maybe we need to implement the fully functional polls.

Let's do a small survey using the new feature.

1. What is your age range?

   • under 16:
   • 16-20:
   • 21-25:
   • 26-35:
   • 36+:

2. Tabs or spaces?

   • tabs:
   • spaces:

3. Have you had covid??

   • yes:
   • no:
   • I don't know:

4. Lately, there have been a lot of meaningless blogs on Codeforces. Is it worth strengthening measures to fight against them?

   • yes definitely:
   • rather yes, but not necessarily:
   • rather no, I read some of them:
   • no definitely:

Hello Codeforces!

Oh yes, 2021 ends! I recently realized that 2022 is pronounced like "2020 too". Not that. I want a completely different year!

Anyway, we did a nice job this year. I hope we have pleased you with interesting problems. Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2022!

I wish you all interesting tasks and brilliant solutions! I wish you error-free code and wish us only rated rounds :-)

Hello, 2022!

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.

Again, this year if you took part in at least 10 rounds you can request a handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 180 days, this period should be in 2019 or earlier. The inactive participant can't have posted comments, messages, and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. A user has the opportunity to request back his/her handle: in this case, we will roll back the change and return your previous handle to you. If you can't change your handle to another, it means that some requirements don't meet. Please do not ask me to do something with it. Especially, I do not like requests like "this is my second account, I took an interesting handle myself". I'm not Santa Claus.

Talking about handles I always
reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

The traditional magical tab has appeared in the profile setting. Happy New Year!

In 2021 (as in 2019 and 2020), with the support of XTX Markets, 6 rounds of the new Codeforces Global Rounds are held: common rounds for both divisions of 7–9 problems each. The duration of the rounds is 2-3 hours, depending on the number and complexity of the problems. All such rounds are rated for all participants. At each such round, 50 brand T-shirts are handed out, and we are happy to give T-shirts to all problem authors.

The prizes for the 6-round series in 2021:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the five rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Current standings after five rounds (sorted by total points in four best placed rounds):

Hello, Codeforces.

Please, welcome c++20 support on Codeforces. Yes, it is 64-bit. Thanks to Brecht Sanders: I used his distribution GCC-11.2.0-64 from https://winlibs.com/.

If you have installed PBOX, you can add this compiler with the line pbox install gcc11-64-winlibs. Probably, a good idea is to add C:\Programs\gcc11-64-winlibs\bin into the PATH. More about PBOX you can read here.

I use the compilation command line similar to other GCC installations: g++ -Wall -Wextra -Wconversion -static -DONLINE_JUDGE -Wl,--stack=268435456 -O2 -std=c++20 <source>. The only differences are -std=c++20 and -Wall -Wextra -Wconversion (I plan to use somehow such warnings in Polygon to suggest fixes in uploaded files).

Now you can use c++20 in your solutions. I'm not sure there are many features useful in competitive programming. Probably, I'm wrong. For example, now you can write vector v{vector{1, 2}}; instead of vector<vector<int>> v{vector<int>{1, 2}};. What else is useful? Please, if you are good with modern C++ then write.

You might be interested in looking at such a table. Before implementation, I always test every C++ distribution for the efficiency of reading and writing large amounts of data. For example, the latest GCC compiler from MSYS2 is terribly slow in some cases. I don't want to use it here. Also, it happens that some specifiers like lld or Lf work unexpectedly. In the table by reference, the second line is the added compiler. The columns correspond to different tests. The cell contains the time of the test execution. If I have time, I will someday publish scripts for testing c++ compiler installations.

Bye for now,
— Mike

Hey! I'm happy to report that Pypy3 is supported. Please test that it works as expected. Thanks!

P.S. I remember about GCC 11, it is in the progress. See you soon.

Hello.

Sorry for it. Actually, right now I don't have any idea why testing is so slow now. Obviously, it doesn't work as expected. It is not because of problems. So please don't blame writers or coordinators. I'm investigating the reason, but don't see it now.

I myself am very upset and I apologize to the authors, coordinator, and participants. I will do my best to find the cause and fix it. Surprisingly, we haven't made any big changes to this part of the system lately. But something went wrong.

Mike.

UPD: https://mirror.codeforces.com/blog/entry/95081#comment-841873

Hello Codeforces!

I am in the process of making improvements and updates to the judgment machines. I've read the post https://mirror.codeforces.com/blog/entry/94587 and I think, maybe it is a good idea to make such a compilation line -O3 -funroll-loops -march=native -mtune=native? I haven't done any research that it is definitely better than -O2 and it is best in the general case for CP solutions. In a way, this will only strengthen the gap from Python/PyPy/Java, on the other hand: in pragmas and so you can set up everything. What do you think? What are suggestions to the command line?

P.S. You got it right. Yes, gcc11 std=c++20, pypy3 64-bit and more are coming.

Hello, Codeforces.

We want to run a series of load testing in the production environment for running contests. For this reason, Codeforces Round 739 will be temporarily extended (in order to test it at the current round). We will temporarily disable the ability to send solutions to it so that its extended duration will not affect the results. After the completion of the work, the duration of the round will be returned.

Our current plan is to finish today.

Mike

Hey!

As I don't see the post discussing the round, I decided to write this.

I do not participate much in contests now. But I love Code Jam. Thanks to problem writers and organizers!

It seemed to me that this round was no more difficult than the previous one. How do you like it?

I liked the problems:

• A (tags: interactive, constructive): strange problem — the most naive approach fits into the requirements on the total cost of requests;
• B (tags: number theory, dp): I wrote some kind of DP with memorization of $$$(a, s)$$$, where $$$a$$$ is the last element in the sequence we already placed (I build in increasing order) and $$$s$$$ is the total sum for the future elements. I don't have a proof why it works fast, probably because of the number of divisors is small.
• C (tags: combinatorics, recurrence): the last occurrence of 1 is the position of $$$n$$$, so we can divide sequence with this position on two parts and use DP to calculate the answer (don't forget to multiply on binomial coefficient);
• D (tags: graphs, flows, greedy): I used min-cost-flow to match Ms on the first field and the second in optimal way. I tried all possible values of flow to iterate over all possible pairs of Ms I want to match and took best cost among all choices.

Hi Codeforces!

Yes, I know it's already April. Time flies so fast lately! Exactly a year ago we were just learning all the "delights" of lockdown.

So let's remember this 2020! What was it like? I don't know about you, but we had a hell of a lot of work.

I will now show you some graphs and you will be very surprised. Lockdown effect? Of course, but we did our best too.

Many thanks to everyone who did not stand aside! In 2020, we celebrated our 10th anniversary and crowdfunding money helps us grow and move forward. You helped us and each other. Thank you. Thank you for your contribution!

We've been working hard all year to make this community a better place. Did it work out? It's up to you to judge. Many thanks to everyone who invested time and effort:

• geranazavr555, kuviman, cannor147 helped a lot with development and new features;
• Una_Shem provided great organizational assistance;
• our problem coordinators are the greatest: KAN, antontrygubO_o, isaf27, 300iq, significant assistance was provided by BledDest, adedalic and budalnik, vovuh for preparing Div3 rounds;
• stingray advised and helped set up the network;
• our friends from VK for their help in our infrastructure;
• and help of problem writers and testers is immortal!

Time to take stock of 2020.

Partner Events

We are pleased to hold programming competitions with companies or for companies. I'm sure this is a great way to support the community of young programmers and hire talented candidates. Here is a list of our main partners this year:

• Telegram and personally Pavel Durov is supporting Codeforces activities for many years, every regular round is held with their help, thank you!
• ICPC — in 2020 we held several joint events (and we liked it!), I hope there will be more
• Mail.Ru — Technocup and Russian AI Cup
• VK, VK Cup — now a personal multi-track competition for Russian-speaking programmers
• XTX Markets, Codeforces Global Rounds — a series of 6 rounds with a separate scoring and souvenirs for the best participants
• Harbour.Space University — a series of educational rounds, the selection of summer school Tech Scouts
• JetBrains, Kotlin Heroes Rounds — Kotlin programming language contests
• Huawei — research competitions (marathons) with elements of machine learning
• Raiffeisenbank — round for both divisions with cool prizes
• Grakn Labs — round for both divisions with money prizes
• Microsoft, Microsoft Q # Coding Contest — unusual quantum computing competition

Hello, Codeforces!

I understand that 2021 has been going on for a long time, but here I have picked up a subset of some of the improvements that the Codeforces team made in 2020. Soon I will publish numbers (no, charts) with statistics for 2020. In the meantime, I bring to your attention a list of changes and improvements.

By the way, this is a decent list. This is about half to a quarter of all changes. It's just that other changes are more often somewhere in the internals of the system and are not visible to users. Please read this list. Each item is the effort of someone from the team. Thanks to geranazavr555, kuviman and cannor147 for their efforts. You've made our platform better! Well, by the way, I don't quit programming and many improvements were made by me.

The items on the list are written in a concise and informal form, many of the items I just copied from commit messages from git. If you want more details — ask in the comments, we will tell you! Of course, I forgot to include some improvements in the list.

Hello Codeforces!

This unusual and difficult 2020 has come to an end! What will the new year be like? I wish all of us that we have the opportunity to meet in person so that the contests take place not only online. I ask you to think of something important and good. Let this wish come true!

We have had a large number of rounds this year. I hope we have pleased you with interesting problems. Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2021!

I wish the participants victories over yourself (and over problems!) and development of your skills, interest and curiosity!

Hello, 2021!

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.

Again, this year if you took part in at least 10 rounds you can request a handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 180 days, this period should be in 2017 or earlier. The inactive participant can't have posted comments, messages and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. A user has the opportunity to request back his/her handle: in this case, we will roll back the change and return your previous handle to you. If you can't change your handle to another, it means that some requirement doesn't meet. Please do not ask me to do something with it. Especially, I do not like requests like "this is my second account, I took an interesting handle myself". I'm not Santa Claus.

Talking about handles I always
reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

The traditional magical tab has appeared in the profile setting. Happy New Year!

— MikeMirzayanov

You can view PDF version of the tutorial by the link.

Hello!

ICPC Southern and Volga Russian Regional Contest (Northern Eurasia) 2020 has ended on November 15. This year the competition took place online. 108 teams competed, many of them received an invitation based on the results in Qualification Contest.

On Dec/25/2020 14:35 (Moscow time) will start online-mirror 2020-2021 ICPC, NERC, Southern and Volga Russian Regional Contest (Online Mirror, ICPC Rules).

Hope you enjoy the problems. In this contest I play a role of Cheif Judge and the jury teams consists of ex-participants of ICPC from Saratov and jury members from other cities: adedalic, ashmelev, BledDest, DmitryKlenov, DStepanenko, elena, KAN, kuviman, MikeMirzayanov, pashka, PavelKunyavskiy, Дмитрий Мещеряков, Герман Наркайтис. Many thanks to all of them!

I send special rays of gratitude to testers: Merkurev, Um_nik, romanasa, josdas, budalnik, Perforator, Oleg_Smirnov, IlyaLos, Supermagzzz, Stepavly, Igor_Kudryashov, HolkinPV, Edvard, le.mur!

I invite ICPC teams and individual participants of Codeforces competitions to take part!

Of course, the competition will be unrated. I ask the participants of the official competition to refrain from participating in the mirror and discussing problems until its end.

Since this year the audience of participants was wider (due to the online format), we also selected problems a little more accessible than in previous years. If you are from the top team that claims a medal in the ICPC Finals, then it may be more fun for you to take part in this contest personally.

Good luck!

In 2020, with the support of XTX Markets, 6 rounds of the Codeforces Global Rounds were held. Each round was common for both divisions. All such rounds were rated for all participants. At each such round, 50 brand T-shirts were handed out, and we gave T-shirts to all problem writers!

I say a big thank you to XTX Markets (and personally to Yuri Bedny and Alexander Gerko) for supporting Global Rounds. Without your help, we could not host them! I sincerely admire companies like these who come to support and develop the community that some of their employees come from. It will be great if other companies follow the example and also offer their help. We need it!

The prizes for the 6-round series in 2020:

• In each round, top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Final results (and congratulations!):

Hello Codeforces!

There is something new for you. It seems that now it will be easier to reuse problems from the archive for educational and other purposes.

As you know, problems from past Codeforces rounds can be added to mashups simply by their codes of the form like 1234D (contest ID + problem letter). I myself regularly used this when I taught at Saratov University — it was very convenient to prepare trainings: problems have expected complexity and topics, there are Russian and English statements, you can see the analysis and many solutions. Because of hacks and uphacks, problems in the archive usually have strong tests. Almost all of them are well-prepared. In addition, with the help of user lists, you can easily pick up problems for a group of students that they did not solve.

But there was also an issue: the problems in such trainings were easily googled. And if the majority of adult participants could cope with the temptation to spy on the solution or tests, then beginners sometimes acted dishonestly. I received several requests to come up with something with this flaw. One of the most recent such requests was from Daniel Darooha Sleator (https://en.wikipedia.org/wiki/Daniel_Sleator). Thanks, Danny!

So the work seems to be complete. Meet a new feature on Codeforces: now you can rewrite statements for problems in mashups, completely replacing it with your own. More details about the capabilities of the new functionality:

• a new icon-link appears in the problem list in mashups, by clicking on which there will be a form for creating/editing a new statement;
• you can use the original text of the statement as a template when writing your own: be very careful — you must exactly repeat all the details of a statement so that problems do not formally differ;
• this form compares the set of formulas in the original text and the new statement, and if they differ as sets, there will be a corresponding detailed warning;
• you can specify your own tests from a statement (examples) — here you also need to be very careful so that they are formally correct;
• if you added examples, then solutions will be judged on them first, and only after on official tests;
• you can upload your own image files and use them in statements;
• there is a setting to temporarily disable the use of the new statement (for example, if it is still under development).

Considering that there are already more than 6500 problems in the archive, an excellent perspective opens up for organizing trainings and educational sessions.

I hope this innovation will be useful. Write in the comments what can be improved and/or corrected.

M.

During the round Grakn Forces 2020 many of you can notice that now we highlight diff in statements if they changed. Do you like this new feature?

Also, I'm thinking on the way to automatically notify participants if statements changed. I still do not fully understand how this should ideally be implemented. Sometimes it is true that just a forgotten comma was added. On the other hand, it is almost always better to notify of changes. What do you think?

In 2020 (as in 2019), with the support of XTX Markets, 6 rounds of the new Codeforces Global Rounds will be held. These will be common rounds for both divisions of 7–9 problems each. The duration of the rounds will be 2-3 hours, depending on the number and complexity of the problems. All such rounds will be rated for all participants. At each such round, 50 brand T-shirts will be handed out, and we will be happy to give T-shirts to all problem authors and problem testers.

The prizes for the 6-round series in 2020:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Current standings after four rounds (sorted by total points in four rounds):