You may print PDF-statements: http://assets.codeforces.com/statements/589.pdf

Recently 2015-2016 ACM-ICPC, NEERC, Southern Subregional Contest has been finished. On behalf of jury and hosts I congratulate winners and advancers to the semifinals. Prize places are (horray!):

• 1 place: Nizhny Novgorod State University #1 (Vladislav vepifanov Epifanov, Nikolay KAN Kalinin, Mikhail mike_live Krivonosov)
• 2 place: Innopolis University #1 (Evgeniy savinov Savinov, Sergey sokian Kiyan, Alexander map Mashrabov)
• 3 place: Saratov State University #1 (Edvard Edvard Davtyan, Vitaliy kuviman Kudasov, Danil danilka.pro Sagunov)

On Saturday (October, 17) в 08:00 (UTC) we will host unofficial online mirror. Interesting problems are waiting for you. Judges tried to prepare problems of wide difficulty range: for newcomers and for expirienced teams. This will be a team/personal contest on Codeforces, with teams consisting of up to three people or individual participants. The contest will not affect Codeforces ratings.

For sure, it will be unrated contest. We recommend you to take part in teams. I think, the contest will be moved to Gym later.

Good luck!

MikeMirzayanov, head of judges.

P.S. We are aware that this mirror will overlap with some other online tournaments, but unfortunately we are unable to change the current time slot, due to the online mirror of the Moscow Subregional of NEERC scheduled on Sunday. All school teams are recommended to pay attention to Online-mirror of the Ural regional team championship.

Hello Codeforces.

I'm honored to announce you, that Codeforces round #326 is gonna take place soon on Codeforces.

Writers are AmirMohammad Dehghan(PrinceOfPersia) and Ali Haghani(Haghani). There will be 6 problems in each division(4 problems are common) and you'll have 2 and a half hour to solve them. I hope you enjoy the problems.

I wanna thank Maxim Akhmedov(Zlobober) for his great helps in preparing this round, MikeMirzayanov for wonderful platforms of Codeforce and Polygon, Delinur for translating problem statements into Russian and MohammadReza Maleki(mruxim) and Ali Bahjati(LiTi) for their great advices (and LiTi again for some graphics).

This is my third round on Codeforces and not the last one, I hope.

This is the last round on Codeforces with Zlobober as coordinator. It was nice working with him. We had a lot of fun and interesting contests during his coordination. I just wanna say: Thank you Maxim for all your contribution to CodeForces community and good luck in the future. After I heard these news, my first guess for the next coordinator was I_love_Tanya_Romanova; But I don't know if CodeForces hires people from out of Russia or not.

The main character of this round is Duff, but you'll also have to help her friend, Malek!

I wish you all high ratings and lots of joy.

Scoring will be posted soon.

GL & HF!

Problems are sorted by the expected difficulty, but we strictly recommend you to read all the problems.

UPD: Scoring is: 750-1000-1500-2000-2500-3000 in Div.2 and 500-1000-1500-2000-2750-2750 in Div.1 .

UPD1: Editorial is published.

UPD2: System test is over. Congratulations to all winners.

Div.1 Winners are:

1. qwer1561
2. Endagorion
3. jcvb
4. subscriber
5. wjh720

Div.2 Winners are:

1. sleepy_mario
2. BIT-silence
3. Owaski
4. Orenji.Sora
5. JavaInTheSouth

Also congratulations to JoeyWheeler, the only one who solved problem Div.1 D.

Hi, Codeforces!

Codeforces Round #325 will take place on October 12, 2015 at 09:00 UTC for the first and second divisions. Please note that the round will start at unusual time!

The problems of this round are taken from the Regional stage of the All-Russian school team programming olympiad that will be at the same time in Saratov, Russia. Lets wish school teams good luck on competition!

The problemset for school olympiad differs from problems that will be on round. In addition in problemset for school olympiad there will be some problems, which are not included in the round and vice versa. So don't be frightened by the difficulty of problems given for school teams :-)

The process of problems preapring was very interesting: we were reworking problem statements many times, adding tests, changing limits, even managed to change fully prepared problem with another (we had to stop the conveyor already printing problem statements :-)) Let's thank all who were preparing, helping in preparing, reading the problem statements, writing solutions: Adilbek adedalic Dalabaev, Roman Roms Glazov, Vladimir vovuh Petrov, Oleg Oleg_Smirnov Smirnov, Alexey Perforator Ripinen, Maxim Neon Mescheryakov, Ilya IlyaLos Los, Vitaliy gridnevvvit Gridnev, Danil danilka.pro Sagunov, Alexander fcspartakm Frolov, Pavel HolkinPV Kholkin, Igor Igor_Kudryashov Kudryashov, Elena elena Rogacheva, Dmitriy Nerevar Matov, Vitaliy kuviman Kudasov. Chairman of the jury of the Olympiad is Mikhail MikeMirzayanov Mirzayanov (also he is the author of some of tasks). I (Edvard Edvard Davtyan) prepared some tasks and coordinated the work of the authors. So we are a large team of writers (I hope I did not forget anyone)!

Also let's thank Max Akhmedov (Zlobober), The-One-Who-Must-Not-Be-Named (if I am not wrong, he or she is right now solving the round problems) for their help in problems preparing, Maria Belova (Delinur) for translating problems in English and again Mikhail Mirzayanov (MikeMirzayanov) for the Codeforces and Polygon systems.

You will have only two hours to solve six problems. The scoring distribution will be anounced a little before the round. I wish high rating to all participants! Good luck and have fun!

P.S.: Also I want to wish good luck to the participants of the ACM ICPC Southern Subregional Programming Contest that will be held on Wednesday.

UPD Due to technical reasons round is delayed by 10 minutes.

UPD: In problem Subway roller there were tests with trains of length one. At this moment authors are discussing how much it troubled the round results. We are apologize to participants who had problems with that. We will announce our decision about this problem soon.

UPD2: There were trains with length equal to one in tests for problem Subway Roller. Jury decided to accept the first right solution of each user that passed all tests except the wrong, also other submissions were ignored. In addition we accept all successed hacks, although the hack is not correct and the hacked solution passed all other tests. Contest will be rated, but any user who think that this situation is cardinally affected to his/her result can send me in 24 hours a message and jury will discuss the question to make round unrated only for that participant. We are apologize to participants one more time for this situation.

UPD3: Editorial

Hello, Codeforces!

I'd like to invite you to Codeforces Round #324 (Div. 2). It'll be held on Tuesday, October 6 at 19:30 MSK and as usual Div. 1 participants can join out of competition.

Great thanks to Zlobober (Maxim Akhmedov) for helping me preparing the contest, to Delinur (Maria Belova) for translating the statements into English, to MikeMirzayanov (Mike Mirzayanov) for the Codeforces and Polygon. This is my first round, and, I hope, it won't be the last one.

You will be given five problems and two hours to solve them. The scoring distribution will be announced later.

Characters from problems have their prototypes — my friends, familiars, native, and this round is dedicated to them.

UPD : 500-1000-1500-2000-2500

Good luck and high rating!

UPD2 Round has finished, thanks for participation!

Congratulations to the winners!

1). Siunaus

2). aasddf

3). M_H_M

4). lal

5). femsub

Editorial will be published later

UPD3 Editorial

In this post I want to describe in short how to write a formulas on Codeforces. In fact it is short introduction to the markup language used on Codeforces.

Three important rules.

Foremost rule: formula me place in dollars ($), as well as in parentheses.

Another important rule: if you want to apply some operation to some group of symbols it is necessary to form the block using the curly braces. For instance, 2^x+y = 2^x + y, but 2^{x+y} = 2^x + y.

Third rule — for perfectionists. For traffic economy Codeforces print simple formulas by usually text. Sometimes it is not very pretty: C_{x_i+y_i-2}^{x_i-1} = C_{xi + yi - 2}^x_i - 1. Is this case you can add command \relax at the beginning of the formula. Then the formula is guaranteed to be beautiful: \relax C_{x_i+y_i-2}^{x_i-1} = .

Arithmetic operations.

Addition and subtraction can be written ordinary symbols + and -. Multiplication is usually indicated by a null character (xy is the produst of numbers x and y) or by symbol (\cdot). If it is necessary to multiply two complex expressions (), or are important both factors, not just the value of the product (in expressions of type field ), use the symbol  × , which may be obtained by the command \times.

The division is somewhat more complicated. Usually in mathematics division is not written in one line, but the desire is not to write the fraction of the blue, too, is understandable. In this case, you can always write a : or / (x:y, x/y).

If you want to write all the same fraction, it has two similar commands: \frac and\dfrac. After any of these commands have to write a block of the numerator and block of the denominator, for example: (\frac{1}{4}). Using \frac small fractions are obtained, which is suitable mainly for the simple fractions. If you want to write a serious big fraction you'll need \dfrac: (\dfrac{x+y}{x^2+y^2}). If the numerator and denominator of a single-character, it can not be enclosed in brackets, for example: (\frac14x), but only if the numerator is not a letter.

The upper and lower indices.

If you want to write a lower index, you will help symbol _ and upper index (basically it is the exponent), the symbol^: (x_i + y_i)² ((x_i + y_i) ^ 2). Same manner as with the fractions in the lower or upper index block can be placed, but if a single-character index, it can not do so.

Other useful tips and special characters

Text — text (---) — not in the formulas, and in the text. This dash, not hyphen (it is not works without surrounding text)
(\dots) — three dots symbol.
∞ (\infty) — infinity symbol.
(\in) and (\ni) — element of the set.
 →  (\to) — arrow right, in expressions such as x_n → 0.
Many well-known mathematical functions can be typed with the '\', then they will look like a formula, rather than simply as text ( = \tg, = \ln, = \lim and so on)
If you want to index are top and bottom rather than the top-right and bottom-right, use the command \limits:
= \sum_{k = 0}^nx^k
= \sum\limits_{k = 0}^nx^k.
If the brackets around a large expression small, you can make them suitable size, written before the left bracket command \left, and before right bracket command \right. For instance: = \left( \dfrac{x+y}{x^2+y^2} \right).

Thank you for the attention!

Hi everybody!

Hope you liked our Revolution of Colors and Titles. Maybe you even have took part in a contest in new status. The last round have set an incredible record: 8000 participants! And I'm glad to tell you that there was no single technical issue during the round time! Consider the 15-minute delay as a part of our evil plan for setting up a new record :)

I'm glad to tell that Codeforces team is able not only to tune colors and formulas, but also to work on new features for you. You may see on contests page that there is a list of authors for each of the rounds! Moreover, in profile of a person there is now an entry called "problemsetting" that allows you to see the list of all contests in whose preparation a person took a part.

Endagorion looks like this.

Greetings, Codeforces!

My name is Danil Sagunov, and I used to be red... Anyway, I wish to congratulate you with the Second Revolution!

I am glad to introduce that this Saturday, 3rd October at 19:30 MSK Codeforces Round #323 for both divisions will take place. Problemset has been prepared for you by me and Vitaly gridnevvvit Gridnev. This is not the first round we are authors of and I'm sure that it's not the last.

Special thanks to our friends Maxim Neon Mescheryakov, Vladimir vovuh Petrov and Roman Roms Glazov for helping us preparing the round. I would like to thank Max Zlobober Akhmedov for his coordinator activity, Michael MikeMirzayanov Mirzayanov for creating and supporting Codeforces and Polygon systems, making this round possible, and Maria Delinur Belova for translating problem statements into English.

Thanks to Vladislav winger Isenbaev and Alex AlexFetisov Fetisov for testing the round problemset!

Both division participants will be given five problems and two hours to solve them. I hope that everyone will find the problems interesting and many of you will return your colors.

UPD The round duration was written incorrectly in the e-mail announcement, the correct duration is 2 hours, not 2.5 hours.

UPD2 Round have been successfully finished! Thank you all for participating.

Congratulations to 1 division winners!

1. ecnerwala
2. ikatanic
3. uwi
4. PavelKunyavskiy
5. sd0061

And the second division:

1. wrong_order
2. ahwhlzz
3. kefaa

My gratitude to fotiIe96, one to solve problem D!

UPD3 Editorial can be found here

Over the last several months the Codeforces team has been looking anxiously at the inflation in the rating caused by both an influx of new users and the imperfect calculating formulas.

In the end it lead to noticeable shifts in colors and titles. For example, getting red in 2015 became much easier than in 2013.

We conducted a survey about the way to introduce the color bounds. We are happy to announce the updated colors and titles!

A summary of the main changes goes like that:

• a new color: greenish-blue color or cyan — just like the name implies, this color takes its place between green and blue, now the participants of the second division will be better differentiated;
• all the colors shift upwards along the rating scale — see the table below. Now reaching the top positions will be harder;
• the legendary grandmaster — the new title and color for those who reached the sky high rating 2900.

The cold colors (gray, green, cyan and blue) still correspond to the second division and the other ones correspond to the first one.

The requirements to the coach rights haven't changed — you've got to have 2200 or 1900 and a rich history of participations in order to become a coach. Problem tags and groups can be added by the participants from the first division.

The table below illustrates the new values of the color and rank borders:

Active users (who took part in last 6 months) on the moment of October, 1 are counted.

I will write about the changes in the rating formulas as soon as possible, And yes, the formulas will be open!

Hooray! Soon there will happen several updates on Codeforces affecting rating and colors. The second Revolution of Colors and Titles is coming!

You will soon find out a new rating with public formula, new color bounds and even something more...

New bounds will fix the rating inflation of last year, it will return exclusivity of high rating and titles. Don't worry if you end up with lower color; it's a new reason for you to move forward!

While discussing the ongoing change we faced an issue about how to apply new colors with two possible solutions.

First solution: forward without looking back.

When applying new bounds we will update colors everywhere according to a new schema. For example, somebody can possibly lose not only the read color, but he may also regain a new challenge "to become red" since he lost time when he was red before on his rating history. For the first time it doesn't seem as a good solution, but if you think deeper, there is nothing bad in it. Ratings will be whole without cases like "you were read before the revolution, that doesn't count!". In old ranklists there will be no mess within colors of modern contestants (from upsolving or virtual contests) and historical contestants. Irregular visitors of Codeforces won't be confused of the fact that somebody is red in the standings but shouldn't have been red before according to his rating.

We did like this before it worked not bad, if we have changed the color history back then, it would have added mess and confusing.

Second solution: keeping history.

When adding new bounds we will keep old ranklists, posts and comments "as is". This won't break comments like "Congratulations with a red color!" (not sure if they are that valuable) Such solution will keep your achievement "to become red" unlocked, i. e. this part of your biography remains still confirmed by your rating history.

We may even make a rating graph become cut on two parts with a vertical line at the moment of a revolution. The old colors will remain to the left of it, and the new colors will be placed to the right of it. Although this may lead to a confusing of newcomers unprofessionals and rare Codeforces visitors.

Overall

As you see, both solutions have their pros and cons. I don't even know what is better among this two possibilities. That's why I want you to vote for the better choice. If one of the solutions surely wins, we'll use it. Otherwise I'll do as I think it is best.

Please vote only after carefully reading both options. First two comments below correspond to the solutions. Negative voices won't be considered at all, positive will have weights 1-2-4-8-16-32 according to your color (grey-green-blue-purple-orange-red). The vote is secret, results will be available on October, 1st in the evening.

UPD: The voting is finished. We congratulate the first option with a confident victory: 6394 points vs 2320 points! The comments for voting have been removed not to affect comment votings statistics. You may expect changes in colors and ranks in the nearest future!