I almost copied my previous blog post to separate two discussions: about Div. 4 in general and about Round 640. Please, visit this blog post to discuss Div. 4 rounds.

Hello Codeforces!

Yes, it is not a mistake I decided to run the first Div. 4 round: Codeforces Round 640 (Div. 4)! It starts on May/09/2020 17:35 (Moscow time).

The format of the event will be identical to Div. 3 rounds:

• 5-8 tasks;
• ICPC rules with a penalty of 10 minutes for an incorrect submission;
• 12-hour phase of open hacks after the end of the round (hacks do not give additional points)
• after the end of the open hacking phase, all solutions will be tested on the updated set of tests and the ratings recalculated
• by default, only "trusted" participants are shown in the results table (but the rating will be recalculated for all with initial ratings less than 1400 or you are unrated participant/newcomer).

I urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism, but simply to take part unofficially. Please do not spoil the contest to the official participants.

Only trusted participants of the fourth division will be included in the official standings table. This is a forced measure for combating with unsporting behavior. To qualify as a trusted participants of the fourth division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1600 or higher in the rating.

Regardless of whether you are a trusted participant of the fourth division or not, if your rating is less than 1400 (or you are a newcomer/unrated), then the round will be rated for you.

Please, read some of my thoughts about the problems of this round:

• Don't expect A+B level problems :-) It is a real contest with easy but problems. I hope most problems fit in range D3A-D3D problems.
• I'm not sure about order of problems. Try to read all problems or at least most of them. Testers expressed skepticism about the order of the problems but suggested different pairs of problems to swap.

We are just starting, so the complexity of the problems may deviate somewhere from your or my expectations. Wait and see.

The first "thank you" is sent to antontrygubO_o, who helped with the review and selection of problems.

Many thanks to the testers: pashka, 300iq, vovuh, Golovanov399, spookywooky, kocko, Sho, igor_kz, Sho, scott_wu, bfs.07.

I hope you enjoy the problems, and the round will be interesting!

Please, leave in comments your thoughts about exactly this round. If you want to write something about the general idea of Div. 4, then visit this blog post.

UPD: Editorial is published!

Hello Codeforces!

Initially, here was an announcement of the first round for the fourth division. But currently, in comments there is the huge discussion of division 4 rounds in general. Honestly, I read all (almost all) comments.

I decided to replace the text of this post to match the comments better. Also, I'll write one more post separately to discuss problems. And this post will be about division 4 rounds in general.

About Div4 rounds:

• I'm not afraid of a queue or servers overload: it is technical challenges and my task is to resolve them. The previous biggest rounds were not overshadowed by technical problems and judging delays, they went well. I see no reasons why something should work much worse in the rounds for Div. 4 (and if it does, then this should be fixed).

• I don't think that Div. 3 rounds are too dificult. Actually, they are quite good for beginners. They are noticeably easier than Div. 2. Almost every round trusted participants solve all problems, many participants solve all without one problem. I think in general they are OK.

• Now I don't like the idea to run in parallel Div. 3 and Div. 4 rounds (like we do for Div. 1 and Div. 2). Div. 3 rounds are harder to prepare and I don't think we can host them more than once per ~2 weeks. It means that such rounds will not give new contests for newcomers and grays/greens (I think Div. 3 are already interesting rounds for them to take part). But separate Div. 4 rounds will be easier to prepare (even than Div. 3) and probably making them we can offer more contests for low rated participants.

• I'd like to repeat: Div. 3 and Div. 4 rounds doesn't affect the rate of Div. 1 and Div. 2 rounds. The coordinators focus only on Div. 1 and Div. 2 rounds. All Div3 (and future, if any, Div4) rounds are prepared without any help from the coordinators.

• I do not think that by hosting Div. 4 rounds, we will lower the quality of problems. Now, I don't plan to use absolutely A+B level problems even for the Div. 4. I think problems of level like Div3A-Div3D are good candidates.

Thanks for your attention to this idea!

Hello, Codeforces!

Paul stingray Komkoff is my hero! He found some issues in the network configuration. The most important of them was about flow control auto-negotiation: in case of our network routers the number of dropped packets reduced in times. Thanks!

To be sure that it really helped to fix the issue I ask you to take part in Testing Round 16 (Unrated). It will start on May/07/2020 17:05 (Moscow time). Probably, it will contain 3 easy problems which are not prepared for now (I'll use some old problems, I think). The only reason to host the round is to check the system under a load. It will be an unrated round.

I am upset about what happened with the last round. Once again, I apologize to both the participants and the writer, coordinator and testers. I wrote Monogon about the incident and apologized.

I am sure that the only way to absolutely prevent unrated rounds is just don't host them. And this is not my way. The most popular rounds were hosted without noticeable technical problems. And I'm sure the main records are yet to come. I ask the writers and coordinators to look forward with optimism and try to please us with problems.

Please, join to the testing round. See you!

— Mike.

UPD: Thank you for testing! I think the system worked great. It seems recent reconfiguration improved system performance and stability. We have some rare "Judgement failed" after database update (I'll try to fix them asap).

Hello, Codeforces!

I'm very glad to invite you to Codeforces Round 639 (Div. 1) and Codeforces Round 639 (Div. 2). This contest will take place on May/06/2020 17:35 (Moscow time). In both divisions, you will have 2 hours 15 minutes to solve 6 problems. The score distribution will be announced closer to the start of the contest.

Of course, this round would not be possible without the help of many people, who I'd like to thank:

• isaf27, for coordinating this round, and helping with solution ideas.
• My ICPC teammates Kuroni and strongoier, and coach cnh355, for encouraging me to pursue competitive programming. In particular, I'd like to thank Kuroni for being so spectacularly bad that a barely-orange user like myself actually has a place on our team.
• 300iq, antontrygubO_o, Ari, ffao, fpc_coder, gamegame, Kuroni, entropy07, McDic, richzli, and VilgefortzOfRoggeveen for testing and providing valuable feedback.
• KAN for checking the problem statements, making them even clearer.
• MikeMirzayanov for providing the Codeforces and Monogon Polygon platforms.

I've done my best to write clear problem statements and strong pretests. Don't forget to read all problems, and I hope you enjoy the round. Good luck!

Because I know you all have so many questions, I have compiled an FAQ:

• Q: Is it rated?
• A: Yes.

UPD

Here is the score distribution.

Div. 1: 500 — 1000 — 1500 — 1750 — 2500 — 2500

Div. 2: 500 — 1000 — 1500 — 2000 — 2500 — 2750

UPD Unfortunately, this contest is delayed due to problems in the Codeforces system. It is temporarily scheduled, but I will update this announcement when a final decision on scheduling has been made. It is sad that my contest is delayed, but let's be grateful that the issue was detected early instead of arising in the middle of the round. On the bright side, this may be the earliest announced score distribution in Codeforces history.

UPD The final decision on scheduling is to keep it as is. The contest will take place on May/06/2020 17:35 (Moscow time).

UPD Unfortunately due to the long queue and other issues, the contest is unrated. There was also a bug that gave us trouble in answering your questions. I'm sorry if you asked a question that went unanswered for a long time. I promise we did our best to answer as many questions as we could! Despite all the problems, I hope you enjoyed the problemset!

UPD Editorial

Hello, Codeforces!

Unfortunately, I report that I decided to reschedule this round. I'm afraid that something might go wrong. Some strange database behavior has appeared, which leads to slow operation in completely unexpected places (it never was and should not be). I will work hard to fix it. My recent tests show that this can dramatically increase judging time (and leads to a huge queue). I don't want to risk the efforts of the writer, the coordinator, testers, and your time. Sorry about it, I was trying to fix it for many hours but I need more time. Hope it will be a great round!

Mike.

UPD 1: I temporarily rescheduled it on May, 6. But I'll discuss the date with the writer and coordinator and reschedule it again to the date they want.

UPD 2: Monogon approved that May/06/2020 17:35 (Moscow time) is good new date and time for the round. See you in 3 days!

UPD 3: It seems I've fixed the issue. It turned that it was connected with week ago incident. Now it is 4 AM and time to sleep.

Hi Codeforces!

I’m thrilled to invite you to Codeforces Round 638 (Div. 2), which will take place on May/01/2020 17:35 (Moscow time). It's rated!

There will be 6 problems, and you will have 2.5 hours to solve them. All the problems are written and prepared by me.

Huge thank you to everyone who made this round possible:

• adedalic for awesome coordination, guidance, advice, and more <3

• dragonslayerintraining for extensive testing, feedback, and buffing tests

• awoo, walnutwaldo20, Benq, hocky, Geothermal, isaf27, McDic, bfs.07, iankury, songsinger, Agnimandur, dcshin, chrome, Rods, brian, and Khairy for taking the time to test and provide invaluable advice

• MikeMirzayanov for wonderful platforms Codeforces and Polygon (preparing problems on Polygon is fun!)

• You, for participating!

I hope everyone will enjoy the problems! I have tried to make short statements and strong pretests. Hopefully this contest will be even better than my previous one :)

UPD: Thanks also to darnley for proofreading statements and suggestions!

UPD: The scoring distribution is standard: 500 — 1000 — 1500 — 2000 — 2500 — 3000

UPD: Editorial is here

UPD: Congratulations to the winners! I hope most of you enjoyed the round. If you have any questions, feel free to ask in the comments.

Overall Winners:

1. dreamoon_love_AA

2. KrK

3. peti1234

4. antontrygubO_o

5. I_love_Tanya_Romanova

Div 2. Winners:

1. Fecto_Elfilis

2. czyarl

3. NishimiyaShouko

4. PureVessel

5. Neraxis

Hi Codeforces!

The Hash Code 2020 World Finals took place on Saturday, April 25! 45 teams representing 22 countries qualified to participate in this virtual event. During four hours of intense competition, finalist teams made 2141 submissions. It was a close competition!

Congratulations to the winners of Hash Code 2020!

• 🥇 Past Glory, score: 6,696,284
• 🥈 simplicissimus, score: 6,478,445
• 🥉 ✷code, score: 6,238,462

View the public scoreboard here.

For those who didn't participate in the World Finals but want to try this year's challenge, we have re-opened our competition platform for an Extended Round, which can be accessed via the Judge System. Please note that you must have registered for the Hash Code 2020 Online Qualification Round and formed a team in order to take part in the Extended Round. The Extended Round will stay open until May 14 at 17:00 UTC.

As an additional bonus, we have added the problem from Hash Code 2019 Online Qualification Round on Kaggle! The problem can be accessed via the Playground Code Competition. Please note that you need to sign up for Kaggle to compete. The round will stay active until July 27 at 23:59 UTC. After this deadline it will be still possible to make submissions, however, the leaderboard will not be updated.

Have fun coding!

Your Hash Code Team

Hi!

As many have noticed, sometimes problem ratings were assigned in a strange way that was not consistent with expectations. For example, ratings for complex problems of Div3 rounds were often overestimated. This was mainly due to the fact that high-ranking unofficial participants did not try such problems. It turned out that despite the high rating of a participant, a problem is not solved by the participant, and this fact raised the rating of the problem. It is not entirely correct to take into account only official participants since ratings for difficult problems are sometimes more accurately determined by unofficial participants.

Somewhere in the comments, I've read that problem ratings are set manually. Of course, this is not so. The process is automated, but I start it manually (I will fix it somehow).

I changed the formulas for calculating problem ratings, now they slightly better correspond to expectations. New problem ratings are already available on the website. I don't think they are perfect (but I hope that they are much better). If somewhere ratings obviously are wrong — it would be great to see such examples in the comments.

Thanks!

UPD 1: Thank you for examples of unexpected problem ratings. I'll try to fix them (I don't think that it is possible to fix all of them without manual work) and return with an update.

UPD 2 [May, 2]: I made another attempt to adjust the coefficients, to take into account some facts differently. The ratings are recalculated again. I carefully went through most of the comments and indicated new ratings. Now it looks a little better. I afraid, there are still some issues with some problems. Try to find them and demonstrate them in the comments. Thanks!

Hello Codeforces!

On Apr/26/2020 17:35 (Moscow time) Educational Codeforces Round 86 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Neon Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Special thanks to Mikhail darnley Dvorkin for helping in round preparation!

Good luck to all the participants!

Our friends at Harbour.Space also have a message for you:

Hi Codeforces!

You really went for it in the last Educational Round! We had an all-time high participation of 21750 people :) We are happy to support such an awesome community, and look forward to growing these numbers in the future!

We are searching for diamonds in the rough — driven, talented humans, passionate about technology and design, undefined by nationality, gender and cultural background. We know that no diamond is born polished, so our mission is to identify and support as many talented young individuals as we can, so that they can fulfill their potential and secure the future they deserve.

If you are graduating or have already completed a bachelor's degree, we are waiting for your applications for fully-funded Master's degree scholarships by the link below.

Congratulations to the winners:

Congratulations to the best hackers:

And finally people who were the first to solve each problem:

UPD: Editorial is out

Hello Codeforces.

I'm writing this post to make comments about a situation on the last Codeforces Round and tell the bad news. We did many mistakes and due to them, the round caused much dissatisfaction from the Codeforces community.

Some of our mistakes:

1. Some stupid mistakes in the statements.

2. In the problem D2E/D1C the numbers $$$d_i$$$ were not necessarily sorted, but there were no pretests for that.

3. The checker of the problem D2F/D1D didn't check one of the requirements and due to that $$$8$$$ solutions failed on pretests during the system testing.

4. After the editorial was posted the mistake was found in the main solution of the problem D1E (more details here). Now we don't know the correct solution.

So, the round was very unsuccessful, I'm very sorry about this. I will make conclusions and will try to avoid such situations later. Please, treat with respect to the authors, even in such a bad situation, a big work was done and I hope you enjoyed the problems.

Now about the bad news: due to the wrong checker and the wrong solution, we decided to make Div1 round unrated, Div2 round is still rated, Sorry for all, who had a big positive rating change.

Also, we are making a D1E problem-solving challenge: if you have an algorithm, that can be proven, please share your approach.

So, that was all news, sorry again,

Ivan.

P.S.

If you are angry now, you can use this post to set a dislike, please don't dislike an announcement/editorial.