Greetings Codeforces!

First things first, we would like to thank everyone who participated in the previous nine Kotlin Heroes competitions: Episode 1, Episode 2, Episode 3, Episode 4, Episode 5: ICPC Round, Episode 6, Episode 7, Episode 8, Episode 9, and Episode 10.

Kotlin Heroes is a great way to play around with Kotlin’s features, learn something new, and practice using the language by solving fun problems. It is great for programmers of any level!

We invite you to join the practice round, where you can improve your algorithmic problem-solving skills and complete training problems. In this round, all the solutions are open and you can request hints if you’re stuck.

Watch tourist and ecnerwala tackle the Kotlin Heroes practice round challenges at the ICPC World Finals in Astana.

Here are some things you can do to help you refresh your knowledge of Kotlin and learn more about competitive programming while you practice:

• Review our Kotlin solutions for the Advent of Code algorithmic puzzles.
• Read our competitive programming tutorial.
• Watch the videos from our Kotlin in Competitive Programming YouTube playlist.
• Look at the problems from the previous practice rounds.

On September 30, 2024, the real challenge begins! Kotlin Heroes: Episode 11 will last 2 hours 30 minutes and will feature a set of problems ranging from simple ones, which are designed to be solvable by anyone, to some really tricky ones for seasoned competitive programmers.

Note that the usage of AI-based tools during the main contest is strictly limited. To find out the guidelines for proper AI use, what is allowed and what is prohibited, please read the post "Rule Restricting the use of AI". Improper use of AI tools during the main contest may result in disqualification.

Using automatic translation tools (including but not limited to neural networks and AI) to convert code written in other programming languages to Kotlin goes against the spirit of the competition, so it is also strictly prohibited.

Prizes:

The top three winners will receive cash prizes of $512, $256, and $128 (or rewards of equivalent value), respectively. The top 50 participants will win a Kotlin Heroes T-shirt and an exclusive Kotlin sticker, and all competitors who solve at least one problem will be entered into a raffle for one of 50 Kotlin Heroes T-shirts.

Please note that we are not able to ship prizes to any country, state, province, or territory subject to comprehensive OFAC sanctions, including Belarus, Cuba, Iran, North Korea, Russia, Syria, or the Crimea, Donetsk, or Luhansk regions. To see the list of sanctioned countries and regions, please visit this page.

Best of luck to everyone!

Hello Codeforces!

We are thrilled to invite you to participate in Codeforces Round 976 (Div. 2) and Divide By Zero 9.0, hosted as Divide By Zero 9.0 by The Programming Club, Indian Institute of Technology Indore (IIT Indore). The contest will take place on Sep/29/2024 18:35 (Moscow time).

UPD: The contest time has been updated to Sep/29/2024 18:35 (Moscow time), which differs from the previously announced schedule. Please take note of this unusual timing and adjust your plans accordingly.

You will have 2 hours to solve 6 exciting problems.

The round will be rated for participants with a rating below 2100.

Problem Setters:

The problems for this round have been authored by nishkarsh and me.

Acknowledgements:

We would like to extend our heartfelt thanks to:

• irkstepanov and KAN for the wonderful coordination and support in preparing the contest.

• Golovanov399, wyrqwq, irkstepanov, Error_Yuan, rolandpetrean, magga, mwen, MrDelrus, letsolum, chromate00, and AnanasClassic for their valuable feedback and testing.

• Special thanks to MikeMirzayanov for the amazing Codeforces and Polygon platforms.

Score Distribution (Div. 2):

• 500 — 750 — 1250 — 1500 — 2000 — 2750

We hope you enjoy the problem set and have a great time solving!

Good luck to all participants!

Ciao, Codeforces! We're glad to invite you to take part in Codeforces Round 975 (Div. 1) and Codeforces Round 975 (Div. 2), which will start on 27.09.2024 16:35 (Московское время). You will be given 6 problems and 2 hours and 30 minutes to solve them in both divisions. Some problems will be divided into subtasks.

UPD: the time has been changed to 27.09.2024 16:35 (Московское время), which is different from the time announced before. Please note the unusual starting time.

This round is based on Italian Olympiad in Informatics (OII) 2024.

The problems were authored by lorenzoferrari, wksni and me.

We would like to thank

• KAN for his nutella coordination;
• Alexdat2000 for Russian translation;
• bortoz, collodel, dp_1, franfill, franv, harniver, jamesbamber, Kaey, veluca93, Virv for helping with both this contest and the onsite contest preparation;
• bortoz for pictures in the statements;
• Flamire for VIP testing;
• Um_nik, EgorUlin, konsteros, Mihail_Podyachih, bugrova, Ludissey, Nonoze, Sofapuden, Temmie, jeroenodb for testing;
• MikeMirzayanov for creating Codeforces and Polygon.

Score distribution:

• Div. 1: $$$500 - 750 - 750 - 1500 - (2250 + 750) - (1500 + 1500 + 1500)$$$
• Div. 2: $$$500 - 1000 - 1750 - 2000 - 2000 - 3000$$$

We hope you'll like the problemset!

Update 1: the editorial is here.

Update 2: congratulations to the winners!

TL;DR: Post an interesting Codeforces blog until October 15th and win $800+1.

This blog is inspired by peltorator's Codeforces Month of Blog Posts.

Hi everyone! The last time, this initiative was a gem for me! I've enjoyed every bit of it and the winner in particular — it helped me when I needed it the most. So now I willing to pay back (or forward :D) — by organizing the Codeforces Month of Blog Posts one more time!

Intro

My view is the same as peltorator's one — I am a huge Codeforces fan, it almost replaces me social media, and the blogs section is one of the best parts of it! Many people may be waiting for the best time to publish their blogs — now the time has come!

Rules & Deadlines

The rules are simple:

1. Post something on Codeforces not earlier than 10th of September (00:02 UTC) 2024

We are looking for novel ideas / new views on the topics / tutorials on the topics that we have not seen yet — anything of your choice. It could even be somthing like this comment! — I think you get the idea: write something novel. If I were to judge, one of the best post of the previous peltorator's months was probably his own announcement :D

2. Send me a personal message until 15th of October (00:02 UTC) 2024 with the link to your codeforces blog post

I would appreciate it if you also include a one-sentence explanation at the beginning of your blog post explaining this challenge and a link to the blog post you are reading right now so that more people can learn about it and participate. So the general deadline is 15th of October.

Prizes!

The winner blog post will be awarded with a money prize — $350+1! The runner-up will win $250! The third place will win $150! The forth place will win $50! :)

If you are willing to increase the budget for the winners — also PM me :D

The process of choosing the winner is not decided yet, I may think of two ways: choosing the winner by myself, or the blog post with the most amount of likes. Write your own opinion in the comments sections

Outro

Let the challenge begin! I hope there will be at least one great blog, so my initative won't be left useless :D

801$ is not a small amount — so I believe in you, make something novel, make something that people want!

Maybe we should hold this sort of thing annually? Something like "Codeforces Year of Blog Posts"? Write our ideas in the comments section!

P.S: Check out LI2 Contests!

UPD1: A great friend of mine decided to give an additional $200 for the runner-up! He insisted on remaining anonymous though D:

UPD2: One month left! Additionally, let's thank i_love_penguins for giving a $100 prize for the third place

UPD2: One month left! Additionally, let's thank orz for increasing the budget by $200!

Hello Codeforces!

Greetings from Nottingham, England! We are delighted to invite you to Legend of Robin Hood, a round inspired by our local folklore.

Codeforces Round 974 (Div. 3) will start on Sep/21/2024 17:45 (Moscow time).

You will be offered 6-8 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. After open hacks all accepted solutions will be rejudged on successful hacks.

You will be given 6-8 problems and 2 hours and 15 minutes to solve them.

Note that the penalty for the 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 prepared by ChairmanFMao, Filikec and RobinFromTheHood.

We would like to thank:

1. Vladosiya for brilliant coordination, and for improving all problems.

2. 18o3, cry, ikrpprppp, pavlekn for orange testing.

3. Axial-Tilted, raztun, vgoofficial for purple testing.

4. Alenochka, FBI, macaquedev, Non-origination, SashaT9, umezo for blue testing.

5. gbula, Pa_sha, 1165MOHITSINGHAL, _Hosam for Div. 3 testing :)

6. MikeMirzayanov for Polygon and Codeforces platforms.

7. You for participating in the round!

Good luck!

UPDATE: Editorial is out!

Image by Zmarlen with our thanks!

Good Day Codeforces!

Me, Wansur and Chalishkan are happy to invite you to take part in Codeforces Round 973 (Div. 2), which starts on Sep/20/2024 17:35 (Moscow time). You will be given 6 problems and 2 hours to solve them. One problem is divided into two subtasks.

The round will be rated for all participants with a rating lower than 2100.

The problems were authored by me, Wansur and Chalishkan to solve and alter them.

We would like to thank:

• FairyWinx for wonderful coordination.
• triple__a, sevlll777, bashkort, sstrong for red testing.
• Tima, exhausted, Kihihihi, Aldk, lebowski998, Issa, shenfe1, Nuris, MnTm for orange testing.
• NewLul, vsinitsynav, for purple tesing.
• Oreki., xruto, BallBreaker, Quinx, Tangirkul, sabyr for blue testing.
• fermatt, Shadibay for green testing.
• MikeMirzayanov and KAN for the great platforms.
• Vinton Cerf, and Robert Kahn for inventing the internet.
• You for participating!

Score Distribution: 500 — 750 — 1250 — 2000 — 2500 — (2000 + 2000)

Good luck!

UPD1: Congrats to the winners!

div. 2:

1. EmmaXII

2. hxano

3. Muelsyse_sep006

4. Hexagons

5. Trumling_hasnotime

div. 1 + div. 2:

1. maspy

2. arvindf232

3. Brovko

4. aryanc403

5. E869120

UPD2: The Editorial is out!

It is our team on EJOI 2024 4-th from left is me, 5-th from left is Wansur

We are also very glad that ICPC 2024 will be in Astana, and we wish all participants a good tour!

Салем Codeforces!

We are glad to invite you to follow 2024 ICPC World Finals in Astana, Kazakhstan!

This is the third ICPC World Finals to be held this year. It is the culmination of the 2023-2024 ICPC season, and we have over 140 teams competing for the title of ICPC champion. Advancing from the regional contest, 141 of the world’s strongest students teams of programmers will compete at the 48th ICPC World Championship in Astana!

2024 ICPC World Finals Astana will begin on September 19, 2024 at 11:00 (UTC+5). We are thrilled to invite you to join the live broadcast of the main event of the year in the world of sports programming!

Teams qualified to ICPC WF Astana:

• Official list
• Non-official list with CF rating
• Non-official CPHoF list of teams with names

Some useful links:

• Official ICPC World Finals website
• Schedule of Events
• Live broadcast
• Photo Gallery
• Astana brochure
• Mirror contest
• Teams' submit reactions
• Non-official scoreboard with CF handles from zibada
• Qingyu Chinese Stream at BiliBili

All available broadcasts:

Also, you can observe teams' monitors and web cameras on the separate broadcast before frozen hour. Sent team's hashtag to the chat to see your favorite!

Follow ICPC World Finals Mirror contest of team ksun48 + Petr + KAN:

• Mirror contest stream
• More details in Petr's blog

Сәттіліқ тілейміз, жақсы нәтиже көрсетулеріңізге тілектеспіз! Good luck, enjoy your time at the ICPC World Finals!

It seems that neural networks are working technological wonders. Not long ago, they struggled with even the simplest tasks in our competitions, but now they are reaching new heights that cannot be overlooked.

There is reason to believe that progress will continue, and we can expect further advancements in neural networks within the realm of programming competitions.

For this reason, we are explicitly limiting the use of AI-based systems (such as various models like GPT, Gemini, Gemma, Llama, Claude, and others) for solving programming problems.

However, we recognize that AI can be a valuable tool for learning and coding assistance, so we aim to establish clear boundaries for its use.

Application of this Rule:
This rule applies strictly to in-contest participation. This includes both registered contestants and out-of-competition participants in any rated Codeforces round. If a round is unrated and this is explicitly stated in the round announcement or rules, the rule will also apply. Outside of such contests, AI tools may be used freely for practice, learning, or non-competitive problem-solving.

Permitted AI Use:

• Translation of Problem Statements: You may use AI-based systems to translate problem statements, but you must ensure that the system does not interpret or summarize the statement. Only direct translations are allowed.
• Code Completion Tools (e.g., Copilot): AI-based code completion systems may be used, but only for syntax and minor coding suggestions. Using them to generate the core logic or algorithms for solving problems is not permitted.

Prohibited AI Use:

• You may not input the problem statement, its summary, any excerpt, or a sub-problem into an AI-based system to receive ready-made code or natural language descriptions of the solution.

• You are forbidden from using AI to diagnose or resolve errors based on system feedback (e.g., after receiving a rejected verdict like "Runtime error on test 1" or similar, you are prohibited from asking AI-based systems to help you fix the issue). The use of AI tools that assist with problem understanding, logic creation, or decision-making in a way that substitutes your own reasoning is strictly disallowed.

Guidelines for Proper Use:

• Using AI to generate simple boilerplate code (e.g., input/output functions) is allowed.
• Relying on AI to generate algorithmic logic or the key solution is strictly prohibited.
• If you're unsure whether a particular AI use violates the rules, please consult the competition organizers.

Cheating Detection:
This rule allows us to continue identifying cheating incidents as we did before the AI era. If two contestants’ codes match and the matched code does not exist publicly on the internet prior to the competition round, this will be considered evidence of cheating. This method ensures that AI tools are not used inappropriately to bypass individual effort and maintains the integrity of fair competition.

We will closely monitor the development of AI technologies and will adapt the rules as necessary while balancing fair competition and the benefits of AI-assisted learning.

Good Day Codeforces!

Me and Tsovak are happy to invite you to take part in Codeforces Round 972 (Div. 2), which starts on Sep/14/2024 17:35 (Moscow time). You will be given 5 problems and 2 hours to solve them. Two problems are divided into two subtasks.

The round will be rated for all participants with a rating lower than 2100.

The problems were authored by me with Tsovak's help to solve and alter them.

We would like to thank:

• TheScrasse, Kaey for wonderful double coordination and chess skills.

• Alexdat2000 for Russian translation.

• turmax, BurnedChicken for nutella testing.

• physics0523, Andreasyan, Arayi, zidder for red testing.

• FelixArg, _LeMur_, _RedWine_, Error_Yuan for orange testing.

• c2zi6, GaGeV, alex_2008, CyberCow, kHarsh3715 for purple testing.

• chromate00, Mher777, dilanyan, Zhora_.004, SashaT9, vikram108, a-c, Doctor_Strange_ for blue testing.

• _PhantomDeluxe_, Arpi2007, tigranmatsak_9, Gurgen_Melikyan for cyan testing.

• ishaandas1, Sonya_2009 for green testing.

• gvardanyan062, Binary_Thinker for grey testing.

• tibinyte for colorblind testing.

• MikeMirzayanov for the great platforms.

• Vinton Cerf, and Robert Kahn for inventing the internet.

• You for participating!

Score Distribution: 750 — (500 — 500) — 1500 — 2250 — (1500 — 2000)

UPD1: The Editorial is out.

UPD2: Congrats to the winners!

Div.2:

1. SSKMF

2. kkkksc03

3. achen.80

4. cqbztl

5. yanold

Div.1 + Div.2:

1. jiangly

2. aryanc403

3. Ormlis

4. maspy

5. kotatsugame

On the left, you see Tsovak with TheScrasse at IOI 2024.

On the right, you see me with Tsovak at IOI 2024 (I Owe Ice Cream 2024).

Heyo Codefoworcers!

vgoofficial and I are very delighted to invite you to participate in Codeforces Round 971 (Div. 4), which will start on Sep/03/2024 17:35 (Moscow time). There will be $$$7$$$ problems, with one split into three subtasks, to be solved in $$$2$$$ hours and $$$30$$$ minutes. We encourage you to participate and hope you have fun, regardless of your division!

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 an unrated participant/newcomer).

We urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism but to take part unofficially. Please do not spoil the contest for 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 unsporting behavior. To qualify as a trusted participant of the fourth 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 1400 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), you may choose to participate rated or unrated.

We want to express overwhelming gratitude to the following orzosities for making the contest possible:

• Vladosiya for his awesome coordination and mesanu for reviewing the problemset once again.

• Our MVTs (Most Valuable Testuwuers), satyam343 and Dominater069, for their dedicated contributions.

• The rest of our army of testuwuers: omeganot, nskybytskyi, awesomeguy856, 18o3, Lilypad, chromate00, Sacharlemagne, ntarsis30, Prady, Non-origination, yash_9a3b, ETL, Orange905, MatthewC3297, mathtsai, macaquedev, jcai972, and cj8450.

• MikeMirzayanov for all that jazz.

01100 00000 11000 10011 00111 01000 10010 01001 01110 10100 10001 01101 00100 11000 01011 00100 00000 00011 10100 10010 10010 10011 00000 10001 10110 00000 10001 00011

UPD: The round will be unrated, even though m1.codeforces.com, m2.codeforces.com, and m3.codeforces.com were functioning correctly. While there were issues on the main site (which lasted more than an hour), no participant was able to view the leaderboard, ask a question, or receive an answer.

Editorial