Hello Codeforces!

On Dec/17/2020 17:35 (Moscow time) Educational Codeforces Round 100 (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.

Also thanks to Nikolay KAN Kalinin for one of the problems' ideas.

Good luck to all the participants!

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

Hey Codeforces!

It’s almost the Holiday season, and this year, we have an extra reason to celebrate — this December marks Harbour.Space’s 5-year anniversary!

Looking back, we’re especially thankful for the wonderful partnerships that have made our university what it is today.

Codeforces has been one of our key partners since the beginning, and we would like to thank the community for growing with us for the past 5 years.

You guys are rock stars, and we’re excited to see where the future takes us both.

Best,
Harbour.Space University

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!

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Hello! Codeforces Round #690 (Div. 3) will start at Dec/15/2020 17:35 (Moscow time). You will be offered 6 problems (one of them is split into two subtasks) 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 1600 or higher, can register for the round unofficially. The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

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

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) 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 trusted participants of the third 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 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.

The problems for this round were invented by MikeMirzayanov and prepared by me Supermagzzz and Stepavly

Thanks to MikeMirzayanov for platforms and coordination of our work. Thanks to Sho, kocko, brian, Crazy_hedgehog, manta1130, Rox, Gassa for help in round preparation and testing the round.

Good luck!

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UPD: Editorial is published

Hello everyone!

We are thrilled to invite you to the great event — ICPC NERC Challenge powered by Huawei 12 — 20 December 2020.

This contest is based on some real problems from Huawei Cloud team. The task is simplified, but still very challenging. You need to find a schedule for a variety of realistic problem instances, where the goal will be minimizing two different cost functions. There is a non-trivial tradeoff, decision you have to make, on how to choose what is best for a particular case. You will be provided with examples that can help you to tackle the problem. A good solution may also require diving deeper into the nature of the problem, and creating some dataset of your own. We believe there are multiple approaches one can try here, and some crossover of the algorithms will also increase your chances to win. Good luck, we hope this will be fun!

The ICPC NERC Cloud Scheduling Challenge powered by Huawei will take place online during the week of December 12-20. This round is unrated for all participants. Everybody is welcome to participate. It is an individual competition. Participants will be divided into two groups: Northern Eurasia Contestants 2020 (all subregions) and all other Participants. If you are a NERC 2020 contestant and you want to take part in ICPC NERC Challenge, link your ICPC account to the Codeforces account here: https://codeforces.net/settings/general

The top-scoring participants will also receive great prizes. Huawei will provide prizes to the 12 winners in both groups accordingly:

• 1-4th place — HUAWEI P40 PRO
• 5-8th place — HUAWEI MATEPAD PRO LTE
• 9-12th place — HUAWEI WATCH GT 2E

In this video Andrey Tikhonov, the co-author of Cloud Scheduling Challenge will explain the basics of the task and describe some special cases and examples:

UPD: ICPC NERC Cloud Scheduling Challenge successfully finished! Thanks all for competing. Congratulations to winners with excellent results! In the video below Andrey Tikhonov will explain the problem solution:

Good luck to all participants!

Hi Codeforces!

I am glad to announce and invite you to Codeforces Round 689 (Div. 2, based on Zed Code Competition), which will be held on Dec/11/2020 17:35 (Moscow time).

We want to offer you to solve 6 problems taken from the Zed Code Competition 2020, which was held as part of the Adapt by Zed conference powered by EPAM Systems.

This round will be rated for the participants with rating lower than 2100.

• The authors of the round are: Aliaksandr AleXman111 Kryvasheyeu, Aliaksandr sdryapko Drapko, Aliaksei netman Vistiazh and me.
• The round was coordinated by Vladimir vovuh Petrov and Nikolay KAN Kalinin.
• The round was tested by: awoo, mohammedehab2002, thenymphsofdelphi, _overrated_, BRCode, morzer, Retired_cherry, mesanu, SlavicG, Shinchan01, manik.jain, Coder-HVZ, Rox, hloya_ygrt, 244mhq, Wind_Eagle, PuRpLe_FoReVeR, Gassa, namanbansal013, saurabhyadavz and andrew.
• And, of course, thank you very much Mike MikeMirzayanov Mirzayanov for creating the polygon and codeforces platforms.

Thanks a lot for your contribution to the preparation of the round! Good luck with the competition!

UPD: The scoring distribution is 500 — 1000 — 1250 — 1500 — 2250 — 2750.

Congratulations to the winners of div 2:

1. yash_daga
2. AiriKatagiri
3. tejas10p
4. RNG-Ming
5. meidong

as well as div 1 winners:

1. neal
2. Geothermal
3. LayCurse
4. emthrm
5. hank55663

Thank you all for participating! (editorial)

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 everyone!

AI Cup 2020 — the annual artificial intelligence programming championship by Mail.Ru Group is already running.

AI Cup — open artificial intelligence programming contest. Test yourself writing a game strategy! It’s simple, clear and fun!

This year's theme is — RTS. We are waiting for you on our site! The site is already open, you can start implementing your strategies.

Ninth AI Cup championship is named CodeCraft. You are to program an artificial intelligence to control a number of units, gather resources, build your settlement and attack your enemies. Your strategies will compete with each other in the Sandbox and the championship. Currently availiable programming languages: C++, C#, F#, D, Go, Java, Kotlin, Scala, Python, Rust. The Sandbox is already open. Good luck!

Schedule

• Sandbox: already open for everyone.
• Round 1: December 12-13.
• Round 2: December 19-20.
• Finals: December 26-28.

Prizes

In the past years, the winners have received prizes, like MacBooks, iPads, watches and hard drives. In addition, we presented 60 hoodies and 360 T-shirts.

Unfortunately, the participants of the previous AI Cup received T-shirts only almost a year later (we delivered the prizes). In a pandemic, we were powerless and not ready.

In this regard, we thought to cancel the merch in this competition, but a the community did not receive this news well. “No promo codes or other virtual prizes needed, we need T-shirts and hoodies” — quote from the community.

Change 1: the prizes are now not physical, but cash. This year's winners will receive:

• 1st place — 250,000 rubles (about $3364).
• 2nd place — 200,000 rubles (about $2691).
• 3rd place — 150,000 rubles (about $2018).
• 4th place — 125,000 rubles (about $1682).
• 5th place — 100,000 rubles (about $1345).
• 6th place — 75,000 rubles (about $1009).

Sandbox winners (also 6 people) will receive 10,000 rubles each (about $135). Amounts are after taxes. We have significantly increased the prize pool.

Change 2: T-shirts and hoodies will be available, but delivery will not be until February. If you don't want to wait, you can exchange your promotional gift for a promotional code in Mail.ru Group services (cloud, etc.).

All participants in Round 2 will receive T-shirts, and the finalists will receive hoodies.

Begin from the Quick start guide. Writing a simple strategy is not hard at all!

¡Buenos días! (That's Spanish for "what's up homies")

On 06.12.2020 17:35 (Московское время) we will host Codeforces Global Round 12.

It is the sixth round of a 2020 series of Codeforces Global Rounds. The rounds are open and rated for everybody.

The prizes for this round:

• 30 best participants get a t-shirt.
• 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.

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.

Thanks to XTX, which in 2020 supported the global rounds initiative!

The problems were written and prepared by smart Cuban Devil and stupid Americans fivefourthreeone and Monogon.

We would like to distribute our thanks equally to the following people who made this round possible.

• Ernestico, McDic, Ari, gamegame, namanbansal013, coderz189, balbit, dorijanlendvaj, Retired_cherry, saurabhyadavz, arvindr9, kclee2172, AliShahali1382, BRCode, ffao, Kuroni for testing the round and roasting providing valuable feedback to the problems. Testers orz!
• antontrygubO_o for inspiration!
• isaf27 for excellent coordination of the round, and improving several problems!
• MikeMirzayanov for the amazing Codeforces and Polygon platforms!
• You, for participating! Your participation will directly contribute to the end of the long-standing Cuban-American rivalry, and will lead to peace on Earth.

You will have 3 hours to solve 8 problems (and 2 subtasks). If you want to lose rating, then we encourage you not to read all the problems.

May rating be distributed from each according to his ability, to each according to his needs!

UPD: Here's the score distribution. Good luck, have fun!

UPD: Hope you enjoyed the problems! Editorial is posted.

UPD: System testing finished, congrats to the winners!

1. Benq
2. tourist
3. jiangly
4. IZONE
5. ecnerwala
6. Um_nik
7. ksun48
8. 244mhq
9. maroonrk
10. yosupo

As you probably know, most of the rounds on Codeforces are created by active community members. We are very grateful to our talented authors from all around the world for bringing their ideas to tens of thousands of competitors, providing fun and learning experience for contestants of every expertise, from beginners to the world's top programmers. This post will cover the steps you need to take propose a round and become a problem writer on Codeforces.

Who can be a problem author

Best problems usually come from the most experienced participants. We have limited resources for rounds reviewing, so to make the process flawless for writers, starting January 1st, 2021, the following requirements for problem authors will be applied:

• rating: 2100 or above, or
• was a writer in the past.

The restrictions may be updated in the future, the post will be updated accordingly. If your round is already in the review status, don't worry, it will be reviewed regardless of the current restrictions.

If you are an experienced writer/jury member in other competitions, please write any current coordinator (listed below) a private message about your wish to prepare a contest and describe your experience in this area.

How to propose a round

If you meet the requirements, you should have the link "propose a contest/problems" in the personal menu on the right sidebar. Click on the link and follow the instructions carefully. Prepare and submit your proposal for a contest and problems.

In some time you will have a coordinator(s) assigned to your contest proposal. They will review your problems and you will discuss the problems with them. Please be active, respond to coordinators' questions in time, make the proposed changes quickly. Codeforces coordinators are very experienced participants and reviewers. Please respect their opinion, and note that they have the highest priority in any decisions on the problems.

Our coordinators are: Aleks5d, errorgorn, irkstepanov, IgorI, Artyom123, 74TrAkToR, ScarletS, Akulyat, TheScrasse, 244mhq, satyam343, Vladithur, and KAN.

You can only propose a whole problemset for a Div 2 or Div 1 + Div 2 round. We don't consider single problems. Also, we don't consider proposals for Div 3/4 or Educational rounds.

How to prepare problems

Problems for Codeforces rounds are prepared in Polygon. It is not recommended to prepare problems before the coordinator's approval, especially if it's the first time you propose a contest.

For each problem you will need to write a clear statement (in English or Russian), code a validator (a program that checks tests' correctness), a checker if necessary (a program that checks output), and of course correct solution. Also, you will need to create tests to check the contestants' solutions on. Your coordinator will do the statement translation if necessary, check your work, and suggests what to change and add if needed.

Before you start working on problems, even if you have experience working with Polygon, you must read these rules. All the work with checkers, validators, interactors, and generators is done with testlib.h library, you can read the introduction by the link and download examples from Polygon. Whenever you have any troubles and/or difficulties, contact your coordinator for help. It is much faster to do something right from the first time, then to do it wrong and then redo it again.

Writers reward

It is interesting and challenging work to invent and prepare problems but is also time-consuming and demanding work. That's why we pay authors for preparing problems for Codeforces. The rewards are:

*If the reward can be paid in cryptocurrency, it might be increased by 25%.

**Sponsored rounds are selected by coordinators from available proposals.

Conclusions

We hope that with your help we will keep the regular stream of regular rounds full of fresh and interesting problems! If you have any questions on how to become a problem writer, ask in the comments. Please note that unrelated or outdated comments in this blog will be cleared once in a while to keep the blog and comments up-to-date.

오랜만이에요, 코드포스! (Long time no see, Codeforces!)

I'd like to welcome all of you to Codeforces Round 688 (Div. 2)! The contest will start at 04.12.2020 16:05 (Московское время), and it is rated for all participants with ratings under 2100. Note the semi-unusual start time.

You will be given 6 problems and 2 hours and 15 minutes to solve them. The score distribution will be announced soon.

All problems are prepared by me, with a lot of help from the testers making me realize that my solutions are dumb.

Thanks to Green55, JooDdae, cs71107, YeongTree, Savior-of-Cross, jh05013, blobugh, 39dll, InfiniteChallenge, Pentagon03, sonjaewon, slah007, jooncco, and kalki411 for testing the round, and especially xiaowuc1 for helping polish English statements as well. I would also like to thank 300iq for round coordination, and MikeMirzayanov for the great Codeforces and Polygon system.

See you in the round!

UPD: The scoring distribution is 500 — 1000 — 1500 — 2000 — 2500 — 3500.

UPD 2: The round is finished. Thanks for your participation! I'm sorry about underestimating the difficulty of problem B, but I hope you still enjoyed the problems! The editorial will be posted in a minute.

UPD 3: The editorial is out!

UPD 4: Congratulations to the winners!

Div. 2

1: caoyizhong

2: Depth_First_Search

3: Misaka23334

4: PleasePreyForMe

5: EzioAuditoreDaFirenze

Unofficial Div. 1

1: Geothermal

2: jiangly

3: neal

4: saketh

5: Pyqe