Vladosiya's blog

By Vladosiya, history, 20 months ago, translation, In English

1800A - Is It a Cat?

Idea: Vladosiya, MikeMirzayanov

Tutorial
Solution

1800B - Count the Number of Pairs

Idea: myav

Tutorial
Solution

1800C1 - Powering the Hero (easy version)

Idea: Vladosiya

Tutorial
Solution

1800C2 - Powering the Hero (hard version)

Idea: Vladosiya

Tutorial
Solution

1800D - Remove Two Letters

Idea: MikeMirzayanov

Tutorial
Solution

1800E1 - Unforgivable Curse (easy version)

Idea: Aris, talant

Tutorial
Solution

1800E2 - Unforgivable Curse (hard version)

Idea: Aris, Vladosiya

Tutorial
Solution

1800F - Dasha and Nightmares

Idea: Gornak40

Tutorial
Solution

1800G - Symmetree

Idea: Vladosiya

Tutorial
Solution

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By Vladosiya, history, 20 months ago, translation, In English

I found a small shortage of materials in English on this topic and I want to start with translated article of rationalex:

The problem

We want to learn how to compare root trees for isomorphism (equality up to the renumbering of vertices + the root of one tree necessarily goes to the root of another tree).

Hash of vertex

Note that since we cannot appeal to the vertex numbers, the only information we can operate on is the structure of our tree.

We will consider as a hash of a vertex without children some constant (for example, 179), and for a vertex with children we will use as a hash some function from the sorted list of hashes of children (since we do not know the true order in which the children should go, we need to bring them to the same form). The hash of the root tree will be considered the hash of the root.

By construction, the hashes of isomorphic root trees coincide (the author leaves the proof by induction on the number of levels in the tree to the reader as an exercise).

Polynomial hash is not suitable

Consider 2 trees:

If we calculate for them to take a polynomial hash as a function of children, we get: $$$h(T1)=179+179p+179p^2=179+p(179+179p)=h(T2)$$$

Which hash is suitable?

As a good hash function, for example, this is suitable

$$$h(v)=42 + \sum_{u \in sorted\_by\_hash(child(v))} log(h(u))$$$

For this hash function, it may seem that it is possible not to sort the hashes of children, but this is not the case, because when calculating floating-point numbers, we have an error, and in order for this summation result to be the same for isomorphic trees, it is also necessary to sum the children in the same order.

An example of a more complicated hash function:

$$$h(v)= \big[\sum_{u \in sorted\_by\_hash(child(v))} h(u)^2+h(u)p^i+42\big]\mod2^{64}$$$

Asymptotics

All we need to do at each level is sorting the vertices by hash value and summing, so the final complexity is: $$$O(|V|log(|V|))$$$

I want to continue on my own:

In the reality of Codeforces, these approaches have problems in the form of hacks (which can be seen, for example, by hacks of this task). Therefore, I want to talk about an approach in which there are no collisions.

What is this magic hash function?

Let's sort the hashes of the children for the vertex and match the number to this array, which we will consider the hash of the vertex (if the array is new, then we will assign it the minimum unoccupied number, otherwise we will take the one that has already been given).

Why does it work fast?

It is easy to notice that the total size of the arrays that we counted is $$$n - 1$$$ (each addition is a transition along the edge). Due to this, even using treemap for mapping, all accesses to it will require a total of $$$O(n \cdot log(n))$$$. Comparing a key of size $$$sz$$$ with another key works in $$$O(sz)$$$ and such comparisons for each key will occur $$$O(\log(n))$$$ times, and the sum of all $$$sz$$$, as we remember, is $$$n-1$$$, so it turns out a total of $$$O(n\cdot \log(n))$$$. (You might think that it is worth using hashmap, but this does not improve the asymptotics and causes the probability of a collision).

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By Vladosiya, history, 20 months ago, translation, In English

Hello! Codeforces Round 855 (Div. 3) will start at Mar/02/2023 17:35 (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.

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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University team: MikeMirzayanov, myav, Aris, Gornak40, senjougaharin and Vladosiya.

We would like to thank: tolbi, lunchbox, Son, sary, Wibo, yorky, nigus, tute7627, 74TrAkToR, bigfoot19982, oversolver, Be_dos, CARBINE, molney, KoT_OsKaR, Nickir, cute_hater, Crutch, vrintle, Rubikun, akulenok, medk, Jostic11, Ghassane for testing the contest and valuable feedback. List of testers will be updated.

Good luck!

UPD: Editorial

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By Vladosiya, history, 21 month(s) ago, translation, In English

1790A - Polycarp and the Day of Pi

Idea: MikeMirzayanov

Tutorial
Solution

1790B - Taisia and Dice

Idea: Gornak40

Tutorial
Solution

1790C - Premutation

Idea: MikeMirzayanov

Tutorial
Solution

1790D - Matryoshkas

Idea: MikeMirzayanov

Tutorial
Solution

1790E - Vlad and a Pair of Numbers

Idea: Vladosiya

Tutorial
Solution

1790F - Timofey and Black-White Tree

Idea: molney

Tutorial
Solution

1790G - Tokens on Graph

Idea: senjougaharin

Tutorial
Solution

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By Vladosiya, history, 21 month(s) ago, translation, In English

Hello! Codeforces Round 847 (Div. 3) will start at Jan/27/2023 17:35 (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.

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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University team: MikeMirzayanov, myav, Gol_D, Aris, Gornak40, senjougaharin and Vladosiya. Also this time, molney suggested one of the tasks.

We would like to thank: alwyn, morasha3, csegura, BledDest, stevenkplus, Darko, Coki628, Crutch, Qwerty1232, Jostic11, liouzhou_101, Jeffin, AW_Flister, glebustim, yorky, mango_lassi, 74TrAkToR, ErrorDeveloper, Be_dos, MODDI, Vercingetorix for testing the contest and valuable feedback. List of testers will be updated.

Good luck!

UPD: Editorial

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By Vladosiya, history, 2 years ago, translation, In English

1759A - Yes-Yes?

Idea: MikeMirzayanov

Tutorial
Solution

1759B - Lost Permutation

Idea: MikeMirzayanov

Tutorial
Solution

1759C - Thermostat

Idea: Vladosiya

Tutorial
Solution

1759D - Make It Round

Idea: MikeMirzayanov

Tutorial
Solution

1759E - The Humanoid

Idea: Gornak40

Tutorial
Solution

1759F - All Possible Digits

Idea: senjougaharin

Tutorial
Solution

1759G - Restore the Permutation

Idea: MikeMirzayanov

Tutorial
Solution

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By Vladosiya, history, 2 years ago, translation, In English

Hello! Codeforces Round 834 (Div. 3) will start at Nov/18/2022 17:35 (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.

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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University team: MikeMirzayanov, myav, Gol_D, Aris, Gornak40, senjougaharin and Vladosiya.

We would like to thank: mumumucoder, Enkognit, orloffm, TeaTime, ilyamzy, Olympia, moonveil, 74TrAkToR, molney, elseecay, bigDuck, Nickir, Be_dos, OAleksa for testing the contest and valuable feedback. List of testers will be updated.

Good luck!

UPD: Editorial

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By Vladosiya, history, 2 years ago, translation, In English

1741A - Compare T-Shirt Sizes

Idea: MikeMirzayanov

Tutorial
Solution

1741B - Funny Permutation

Idea: MikeMirzayanov

Tutorial
Solution

1741C - Minimize the Thickness

Idea: MikeMirzayanov

Tutorial
Solution

1741D - Masha and a Beautiful Tree

Idea: Gornak40

Tutorial
Solution

1741E - Sending a Sequence Over the Network

Idea: MikeMirzayanov

Tutorial
Solution

1741F - Multi-Colored Segments

Idea: MikeMirzayanov, senjougaharin

Tutorial
Solution

1741G - Kirill and Company

Idea: Vladosiya

Tutorial
Solution

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By Vladosiya, history, 2 years ago, translation, In English

Hello! Codeforces Round 826 (Div. 3) will start at Oct/11/2022 17:35 (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.

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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University team: MikeMirzayanov, myav, Gol_D, Aris, Gornak40, senjougaharin and Vladosiya.

We would like to thank: Mangooste, Ormlis, oversolver, Be_dos, IzhtskiyTimofey, Stefan2417, ace5, BledDest, AbsurdMan, toxabuk, Kniaz, goncharovmike, YTatarin, pashka and great_fortune for testing the contest and valuable feedback. List of testers will be updated.

Good luck!

UPD: Editorial

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By Vladosiya, history, 2 years ago, translation, In English

1729A - Two Elevators

Idea: Vladosiya

Tutorial
Solution

1729B - Decode String

Idea: MikeMirzayanov

Tutorial
Solution

1729C - Jumping on Tiles

Idea: MikeMirzayanov, Aris

Tutorial
Solution

1729D - Friends and the Restaurant

Idea: MikeMirzayanov, Aris, myav

Tutorial
Solution

1729E - Guess the Cycle Size

Idea: Gornak40, MikeMirzayanov

Tutorial
Solution

1729F - Kirei and the Linear Function

Idea: Gornak40

Tutorial
Solution

1729G - Cut Substrings

Idea: senjougaharin, MikeMirzayanov

Tutorial
Solution

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By Vladosiya, history, 2 years ago, In English

1714A - Everyone Loves to Sleep

Idea: Vladosiya

Tutorial
Solution

1714B - Remove Prefix

Idea: MikeMirzayanov

Tutorial
Solution

1714C - Minimum Varied Number

Idea: MikeMirzayanov

Tutorial
Solution

1714D - Color with Occurrences

Idea: MikeMirzayanov

Tutorial
Solution

1714E - Add Modulo 10

Idea: senjougaharin

Tutorial
Solution

1714F - Build a Tree and That Is It

Idea: MikeMirzayanov

Tutorial
Solution

1714G - Path Prefixes

Idea: MikeMirzayanov

Tutorial
Solution

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By Vladosiya, history, 2 years ago, translation, In English

Hello! Codeforces Round 811 (Div. 3) will start at Aug/01/2022 17:35 (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 ACM-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 6-8 problems and 2 hours and 15 minutes 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 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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University teams: MikeMirzayanov, myav, Gol_D, Aris, senjougaharin, me Vladosiya.

Also many thanks to yorky, Jostic11, turmax, oversolver, ivanz, antonis.white, molney, KerakTelor, andrey.starodubtsev, Ahmad45123, myway, sofiaasta, Muhammad98 for testing the contest and valuable feedback.

Good luck!

UPD: Editorial

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By Vladosiya, history, 2 years ago, translation, In English

1702A - Round Down the Price

Idea: MikeMirzayanov

Tutorial
Solution

1702B - Polycarp Writes a String from Memory

Idea: MikeMirzayanov

Tutorial
Solution

1702C - Train and Queries

Idea: MikeMirzayanov

Tutorial
Solution

1702D - Not a Cheap String

Idea: MikeMirzayanov

Tutorial
Solution

1702E - Split Into Two Sets

Idea: MikeMirzayanov

Tutorial
Solution

1702F - Equate Multisets

Idea: MikeMirzayanov

Tutorial
Solution

1702G1 - Passable Paths (easy version)

Idea: MikeMirzayanov

Tutorial
Solution

1702G2 - Passable Paths (hard version)

Idea: MikeMirzayanov

Tutorial
Solution

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By Vladosiya, history, 2 years ago, translation, In English

Hello! Codeforces Round 797 (Div. 3) will start at Jun/07/2022 17:35 (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 ACM-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 6-8 problems and 2 hours and 15 minutes 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 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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University teams: MikeMirzayanov, MisterGu, myav, Gol_D, Aris, senjougaharin, me Vladosiya.

Also many thanks to Kirill22 , daubi , Fortin , Artem_Sukharev , vsinitsynav , yorky , oversolver , majorro , ilya_totl , Undying , olya.masaeva , Kniaz , Golovanov399 , farmerboy , Absyarka , Kavaliro , neeraj_joshi for testing the contest and valuable feedback.

Good luck!

UPD:Editorial

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By Vladosiya, history, 3 years ago, translation, In English

1675A - Food for Animals

Idea: MikeMirzayanov

Tutorial
Solution

1675B - Make It Increasing

Idea: MikeMirzayanov

Tutorial
Solution

1675C - Detective Task

Idea: Gol_D

Tutorial
Solution

1675D - Vertical Paths

Idea: MikeMirzayanov

Tutorial
Solution

1675E - Replace With the Previous, Minimize

Idea: myav

Tutorial
Solution

1675F - Vlad and Unfinished Business

Idea: Vladosiya

Tutorial
Solution

1675G - Sorting Pancakes

Idea: Vladosiya

Tutorial
Solution

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By Vladosiya, history, 3 years ago, translation, In English

Hello! Codeforces Round 787 (Div. 3) will start at May/05/2022 17:35 (Moscow time). You will be offered 6-7 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 ACM-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 6-7 problems and 2 hours and 15 minutes 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 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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University teams: MikeMirzayanov, MisterGu, myav, Gol_D, Aris, senjougaharin, me Vladosiya.

Also many thanks to avevad, yorky, UncleSema, vsinitsynav, GoracioNewport, Tvorozh0k, any_nickname, I.AM.THE.WILL and Jostic11 for testing the contest and valuable feedback.

Good luck!

UPD:Editorial

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By Vladosiya, history, 3 years ago, translation, In English

1650A - Deletions of Two Adjacent Letters

Idea: MikeMirzayanov

Tutorial
Solution

1650B - DIV + MOD

Idea: Vladosiya

Tutorial
Solution

1650C - Weight of the System of Nested Segments

Idea: myav

Tutorial
Solution

1650D - Twist the Permutation

Idea: MikeMirzayanov

Tutorial
Solution

1650E - Rescheduling the Exam

Idea: senjougaharin

Tutorial
Solution

1650F - Vitaly and Advanced Useless Algorithms

Idea: Aris

Tutorial
Solution

1650G - Counting Shortcuts

Idea: MikeMirzayanov

Tutorial
Solution

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By Vladosiya, history, 3 years ago, translation, In English

Hello! Codeforces Round 776 (Div. 3) will start at Mar/08/2022 17:35 (Moscow time). You will be offered 7-8 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 ACM-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 7-8 problems and 2 hours and 15 minutes 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 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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University teams: MikeMirzayanov, MisterGu, myav, Gol_D, Aris, senjougaharin, me Vladosiya.

Also many thanks to mango_lassi, espr1t, karemo, starboy_jb, Fly_37, omikad, Katya_Goryachkina, Omja, teraqqq, oversolver, Jostic11, yorky, _4dr_ and doreshnikov for testing the contest and valuable feedback.

Good luck!

UPD: Congratulations girls on International Women's Day <3.

UPD 2: Editorial

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By Vladosiya, history, 3 years ago, translation, In English

1624A - Plus One on the Subset

Idea: MikeMirzayanov

Tutorial
Solution

1624B - Make AP

Idea: senjougaharin

Tutorial
Solution

1624C - Division by Two and Permutation

Idea: MikeMirzayanov

Tutorial
Solution

1624D - Palindromes Coloring

Idea: senjougaharin

Tutorial
Solution

1624E - Masha-forgetful

Idea: Aris

Tutorial
Solution

1624F - Interacdive Problem

Idea: Vladosiya

Tutorial
Solution

1624G - MinOr Tree

Idea: Vladosiya

Tutorial
Solution

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By Vladosiya, 3 years ago, translation, In English

Hello! Codeforces Round 762 (Div. 3) will start at Dec/20/2021 17:35 (Moscow time). You will be offered 7-8 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 ACM-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 7-8 problems and 2 hours and 15 minutes 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 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.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of our work. Problems have been created and written by ITMO University teams: MikeMirzayanov, MisterGu, myav, Gol_D, Aris, senjougaharin, me Vladosiya and SGU student Brovko.

Also many thanks to Geothermal, Rafbill, Sugar_fan, mango_lassi, hitonanode, Resende, Igorjan94, CtrlAlt, KerakTelor, FlakeLCR, MatheusMonteiro, Loolo, kocko for testing the contest and valuable feedback.

Good luck!

UPD 1: The opinion of the testers about the order of problems turned out to be so heterogeneous that there is no way to be sure about the increase in the complexity of problems in the set. We advise you to read all problems.

UPD 2: If you are a schoolchild from the Saratov region, then please refrain from participating in the round. One of the problems may be familiar to you. Thanks!

UPD 3: Editorial

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