Hello, Codeforces!

I'm pleased to invite you to Codeforces Round 901 (Div. 1) and Codeforces Round 901 (Div. 2). It starts on Sep/30/2023 17:35 (Moscow time). It also means, the National Day of China arrives during the round. Wish you a nice vacation! 🎉🎉🎉

For both divisions, you will have 3 hours to solve 7 problems.

I would like to thank:

• errorgorn for his wonderful coordination.

• Alexdat2000 for Russian translation and Vladosiya for helping us with Russian clarifications.

• GeZhiyuan for providing all the problems and "a few" rejected problems.

• znstz2018 and SanweiTreap for helping me prepare the problems.

• Um_nik, wangziji, Heltion, Kevin114514 and flowerletter for Legendary Grandmaster Testing.

• sjcsjcsjc, He_Ren, GaryZ005, platelet, njwrz, thenymphsofdelphi, DerekFeng, Juanzhang and ttklwxx for International Grandmaster Testing.

• maomao90, juju527, zimpha, 765, xuanquang1999, CrTsIr and EasonTAO for Grandmaster Testing.

• SATSKY, lanhf, Hank2019 and Yakumo_Ran for Master Testing.

• DaniraSilla and kriepiekrollie for Candidate Master Testing.

• raulandreipop, micho, alwyn, ArgintLaAnu, IssamOtoz and jakao for Expert Testing.

• CLown1331 for Pupil Testing.

• You for participating the round.

• MikeMirzayanov for Codeforces and Polygon systems.

Especially, I would like to thank errorgorn for his great contribution to the round, njwrz for providing user solutions for all the problems, Kevin114514 for making the announcement and tutorial more readable.

Finally and most importantly, I would like to thank znstz2018 for helping me with everything patiently all the time. This round can't happen without him.

The main character of the problems will be Jellyfish, a sweet little girl. 🍏🍏🍏

Score distribution:

• Div.1: $$$500$$$ − $$$1250$$$ − $$$1500$$$ − $$$2250$$$ − $$$3000$$$ − $$$4000$$$ − $$$5000$$$

• Div.2: $$$500$$$ − $$$1000$$$ − $$$1000$$$ − $$$1250$$$ − $$$2000$$$ − $$$2250$$$ − $$$3000$$$

Good Luck & Have Fun! 🔥🔥🔥

UPD1: Editorial is out.

UPD2: Congratulations to the winners!

Hello, Codeforces!

ognjentesic, wxhtzdy, OAleksa, AndrewPav, Acanikolic73 and I are glad to invite you to the Codeforces Round 900 (Div. 3).

It starts on Sep/26/2023 17:35 (Moscow time)

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

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

Also note that there will be a 12 hour open hacking phase after the round.

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, the round will be rated for you.

Problems have been created and prepared by AlphaMale06, ognjentesic, wxhtzdy, OAleksa, AndrewPav, Acanikolic73, and Vladosiya.

We would also to thank:

1. Vladosiya for the amazing coordination and help with preparing a balanced problemset, and for giving us the opportunity to create a Div. 3 round.
2. MikeMirzayanov for the amazing Polygon and Codeforces platforms.
3. JovanB, abc864197532, sevlll777 for red testing
4. NemanjaSo2005, n0sk1ll, allllekssssa, Alexdat2000, misorin, Hyperbolic for yellow testing
5. DAleksa, ljuba, alex.kudryashov for purple testing
6. MihailoT, SashaT9, ELDRVD, Chrisedyong, tryptophan for blue testing
7. Andrijanikolic73, Mihajlo18, UrosNedic, Klaus26, Escaquejant, smax294, max0n for cyan testing
8. Budimmm for gray testing

Also a very special thanks to the most dedicated tester I have ever seen, NemanjaSo2005.

I wish you luck, and hope you like the problems and learn from them.

Update: Editorial is out.

Congratulations to the winners:

Unofficial:

1. jiangly

2. SSerxhs

3. BucketPotato

4. neal

5. risujiroh

Official:

1. hackerjohn

2. wahb

3. kutcoder

4. Sxy_Limit

5. not_rainboy

Hello, Codeforces!

I am pleased to invite you to participate in Codeforces Round 899 (Div. 2), which will be held on Sep/25/2023 17:35 (Moscow time).

All problems were written and prepared by me.

You will be given 5 problems, and one of them is divided into two subtasks. You will have 2 hours to solve them.

I would like to thank:

• irkstepanov for excellent coordination and Russian translation!

• Golovanov399 for providing useful feedbacks.

• kizen for helping me much with one problem of the contest.

• ko_osaga for Nutella testing.

• leinad2, Numbering, qwerasdfzxcl, kizen, Golovanov399 for Red testing.

• Kaling, Mingyu331, chaeyihwan, Kawaii2DIdolOfGSHS, azberjibiou, mjhmjh1104, DELTARUNE, wuhudsm, hypergraph_coloring for Orange testing.

• unordered_map, alex0211, gs20036, namolbbaemi, eeeDA, nlog for Purple testing.

• andyandy, JaewonOh, KongUm, Coruncanius, ez_code, fs_edge, chromate00 for Blue testing.

• Hanlana18, papergundam, sjmjys954646 for Gray testing.

• MikeMirzayanov for great Polygon and Codeforces platforms!

• And last but not least, You for participating in the round!

Score distribution: $$$500 - 1000 - 1500 - 2000 - (2000 + 2000)$$$

Good luck and enjoy the round!

UPD: Editorial is out.

UPD2: Congratulations to the winners!

Official Contestants

1. Lycorisqwq
2. zepto
3. vjudge39
4. Beelzebub_blue
5. _t_W_

Official + Unofficial Contestants

1. ksun48
2. jiangly
3. antontrygubO_o
4. Heltion
5. KumaTachiRen

Hello Codeforces!

On Sep/24/2023 17:35 (Moscow time) Educational Codeforces Round 155 (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 Adilbek adedalic Dalabaev, Ivan BledDest Androsov, Maksim Neon Mescheryakov, Roman Roms Glazov and me. Also, huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

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

Scholarships for Master's students in Computer Science and Data Science are available in Harbour.Space University Barcelona campus!

Scholarship Summary:

• Fully funded scholarship (29.900 €/year) to study a Master’s degree in Data Science or Computer Science for two years
• Successful applicants will become part of the University’s “Talent pool” and will be shown to the sponsoring companies. In case the candidate is picked for the Work&Study Program from our industry partner, the student starts an internship with a commitment to study 3 hours/day and work for 4 hour/day

Please note preselected candidates will be requested to pay a non-refundable application fee of 85€ to study at Harbour.Space University.

Candidate’s commitment:

You will complete 15 modules (each three weeks long) in one year. The daily class workload is 3 hours, plus homework to complete in your own time.

Candidate’s requirements:

• Bachelor's degree in the field of Mathematics, Statistics, Data Science, Computer Science or similar
• English proficiency
• Advanced knowledge and experience in Python, SQL, Spark/Scala, and bash
• Experienced use of Big Data technologies: Spark, HDFS, Kafka, etc.
• Hands-on experience with Data Science techniques: feature engineering,
• Strong ML knowledge
• Strong Software Engineering Background
• Problem-solving aptitude

UPD: Editorial is out

Hello Codeforces!

flamestorm, MikeMirzayanov and I want to invite you to Codeforces Round 898 (Div. 4).

It starts on Sep/21/2023 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 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 behaviour. 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), then the round will be rated for you.

Many thanks to the testers: Gheal, Phantom_Performer, KrowSavcik, haochenkang, sandry24, BucketPotato, Vladosiya, NintsiChkhaidze, erekle, Dominater069, MADE_IN_HEAVEN, Qualified.

We suggest reading all of the problems and hope you will find them interesting!

Good Luck!

UPD: Editorial is out!

Hello, Codeforces!

SomethingNew and I are glad to invite you to CodeTON Round 6 (Div. 1 + Div. 2, Rated, Prizes!), which will take place on Sep/18/2023 17:35 (Moscow time). You will have 2 hours 15 minutes to solve 8 problems. The round will be rated for everyone.

We would like to thank:

• 74TrAkToR for coordinating this round.
• isaf27, ugly2333, DataStructures, Little09, bashkort, Kieray, fishy15, Vladithur, Dart-Xeyter, pakhomovee, makrav, meowcneil, 127.0.0.1, Nanani, nnv-nick, SomethingDifferent, vl342, shiv_codegen, M1lkas, allmath, sdyakonov for testing the round and providing useful feedback.
• MikeMirzayanov for Codeforces and Polygon systems.

Score distribution:

500 — 750 — 1500 — 1750 — 2750 — 3250 — 3250 — 4000

We hope that our problems will be interesting for you!

Editorial

Congratulations to the winners!

1. orzdevinwang
2. cnnfls_csy
3. jiangly
4. emptyhope
5. maspy
6. tourist
7. AoLiGei
8. Um_nik
9. noimi
10. Geothermal

And here is the information from our title sponsor:

Hello, Codeforces!

We, the TON Foundation team, are pleased to support CodeTON Round 6.

The Open Network (TON) is a fully decentralized layer-1 blockchain designed to onboard billions of users to Web3.

Since July 2022, we have been supporting Codeforces as a title sponsor. This round is another way for us to contribute to the development of the community.

The winners of CodeTON Round 6 will receive valuable prizes.

The first 1,023 participants will receive prizes in TON cryptocurrency:

• 1st place: 1,024 TON
• 2–3 places: 512 TON each
• 4–7 places: 256 TON each
• 8–15 places: 128 TON each
• …
• 512–1,023 places: 2 TON each

We wish you good luck at CodeTON Round 6 and hope you enjoy the contest!

Hello, Codeforces! 🤯

green_gold_dog, ace5, bashkort and I salyg1n are pleased to invite you to participate in Codeforces Round 897 (Div. 2), which will take place on Monday, September 11, 2023 in 17:35.

This round will be rated for participants whose rating is below 2100.

You will be given 6 tasks and 2 hours to solve them. We recommend reading all the tasks. Interactive tasks may occur in the round, so we suggest reading this post.

We want to thank:

• Aleks5d for coordinating the round and improving one of the tasks. 🤓

• Allvik06, Nutella3000, arbuzick, alegsandyr, maomao90, FairyWinx, zwezdinv, valerikk, danya111, EJIC_B_KEDAX, dushenkov, fishy15, makrav, pakhomovee, aleksandro, Yar_sem, asafiul, Ivan_len, ArtAlex, Undying, JustNik77, Pavarishko and xnor for testing the round and giving valuable feedback. 🙏 orz

• ryskmax for being the only blue tester and giving valuable feedback. 🐳

• DimaTomsk and ilyakrasnovv for viewing some of the tasks and giving valuable feedback. 🤠

• stepanov.aa for not testing the round. 🦥 :(

• The authors of the task 5. Overtaking from IOI 2023 for bashkort taking a gold medal. 🥇 Too dead? Yes, so am I.

• MikeMirzayanov and geranazavr555 for the excellent codeforces and polygon platforms. 🦖 Thank you!

We hope you'll enjoy our problems! Good luck!

UPD. Score distribution: $$$500 - 1000 - 1250 - 2000 - (2000 + 1000) - 4000$$$.

UPD2. Congratulations to the winners!

Div. 2

1. candidatecandidatemaster

2. Coreopsis

3. iFFT

4. Dorothy__

5. wfoifysaasisufgv

Div. 1

1. Golovanov399

2. Geothermal

3. arvindf232

4. Rubikun

5. Vercingetorix

UPD3. Editorial.

Hello, Codeforces! & Happy Chinese Teachers' Day!

We (OICon Team) are so excited to invite you to take part in OICon Round 1 (Codeforces Round 896 (Div. 1) and Codeforces Round 896 (Div. 2)) which will start on Sep/10/2023 17:05 (Moscow time). You will be given 6 problems and 2.5 hours to solve them in both divisions.

• Please notice the unusual starting time!
• One of the problems has two versions but their statement are not exactly the same, which means that you can't pass the easy version using the code of the hard version. However, you can only make hacks when you pass both versions.
• There will be $$$0$$$ interactive problem in each division so you needn't read the guide for interactive problems.
• Score distribution:
  • Div. 1: $$$500-(500+750)-1000-1750-2500-3000$$$
  • Div. 2: $$$500-750-1250-(1250+1500)-2500-3250$$$

The problems were authored and prepared by Error_Yuan, duck_pear, programpiggy and me.

We would like to thank:

• Artyom123 for his long-term & high-quality coordination 😊😊😊
• Again, Artyom123 for the Russian translation 👍👍👍
• dqstz, BreakPlus, DisconnectedGraph for providing a few rejected problems 😭😭😭
• Um_nik, gyh20, flowerletter, Heltion for Legendary GrandMaster testing 👽👽👽
• sansen, Kubic, Alex_Wei for International GrandMaster testing 😎😎😎
• rui_er, bilibilitdasc, swiftc, themoon, N_z__, WH6BNNS for GrandMaster testing 🧐🧐🧐
• HappyIvan, Azuma_Seren, User_Carrot, Wilson_Inversion, chen_03, Valters07, Lavine, Kieray for Master testing 😯😯😯
• guoziyang, _YXJS_, Alan_dong, remake_noob_smallpeter for Candidate Master testing 🤔🤔🤔
• Coffee_zzz, fAKezjf, purplevine, Black_Fate for Expert testing 😃😃😃
• mikcf, AIskeleton for Specialist testing 🙂🙂🙂
• wz20201136 for the only Newbie testing 👀👀👀
• ChatGPT for getting 0 point in both divisions 🤡👈🤣
• And, You, for participating and getting positive delta 😘😘😘
• Last but not least, MikeMirzayanov for great Polygon and Codeforces platforms 🎅🎅🎅

Good Luck & Have Fun!

UPD1. Editorial is out.

UPD2. Contest is over! Congratulations to the winners!

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. notyourbae, gs20036, Kolychestiy for purple testing.
7. martin0327, tnaito, TooManyMind, Pa_sha, SashaT9, ctraxxd, ezdp, Nafis for blue testing.
8. NerfThis, _SADIEM_, YudoTLE, sayed_4 for cyan testing.

Good luck!

UPD: Editorial