Привет, codeforces \[^-^]/
На связи лаборатория олимпиадного программирования ИТ-кампуса НЕЙМАРК в Нижнем Новгороде!
Мы рады пригласить вас на Codeforces Round 1013 (Div. 3), который состоится во 25.03.2025 17:35 (Московское время).
Раунд основан на задачах финала первой олимпиады НЕЙМАРК. В олимпиаде приняли участие 312 школьников из 28 субъектов РФ. А на итоговом состязании, кроме нижегородцев мы встретили финалистов из Республики Чувашия, Москвы, Тольятти, Кирова, Саранска и Ульяновска.
Если вы участвовали в финале данной олимпиады — пожалуйста, воздержитесь от официального участия в этом раунде.
Тематика задач будет связана с рабочими буднями и выходными нашей лаборатории. Лаборатория существует чуть меньше года, но за это время было проведено несколько тренировочных сборов, множество личных и командных тренировок, а также разработаны десятки задач на школьные и студенческие соревнования (некоторые из этих соревнований вы можете найти в разделе Тренировки на CodeForces). Мы открыли кружки по программированию в школах Нижнего Новгорода и области, а также начали обучение заинтересованных педагогов-информатиков. Также выступали площадкой проведения нескольких соревнований по программированию.
А во 25.03.2025 17:35 (Московское время) мы, наконец, поделимся с вами одним из важнейших итогов нашей работы.
У вас будет 2 часа и 15 минут на то, чтобы решить 7 задач. Штраф за неверную посылку будет равняться 10 минутам. Раунд будет рейтинговым для участников с рейтингом меньше 1600. Участники с рейтингом 1600 и выше могут зарегистрироваться на раунд вне конкурса. Участники с рейтингом ниже 1600 также могут использовать нерейтинговую регистрацию, чтобы поучаствовать в раунде вне конкурса.
Раунд пройдет по правилам образовательных раундов. Таким образом, во время раунда задачи будут тестироваться на предварительных тестах, а после раунда будет 12-ти часовая фаза открытых взломов, после её завершения все успешные попытки будут перетестированы на успешных взломах (мы очень надеемся, что в течение нее упадет не очень много решений).
Напоминаем, что в таблицу официальных результатов попадут только достоверные участники третьего дивизиона. Как написано по ссылке — это вынужденная мера для борьбы с неспортивным поведением. Для квалификации в качестве достоверного участника третьего дивизиона надо:
- принять участие не менее чем в двух рейтинговых раундах (и решить в каждом из них хотя бы одну задачу),
- не иметь в рейтинге точку 1900 или выше.
Независимо от того, являетесь вы достоверными участниками третьего дивизиона или нет, если ваш рейтинг менее 1600, то раунд для вас будет рейтинговым.
Надеемся, что задачи вам понравятся! Для вас их подготовили сотрудники лаборатории Алексей ashmelev Шмелев, Ислам l-_-l Шаяхметов и Диана Diall_ Алясева.
Мы хотели бы поблагодарить MikeMirzayanov за платформы CodeForces и Polygon, а также Vladosiya за координацию подготовки раунда.
Большое спасибо за тестирование: MForest, OG_Matveychick1, Tmitmi, Riladavin, quaha, eepsilon, anotherworld, konred, NerfThis, gtheoden42, sasha00123, ReshuVse, artem., OG_Sergzhick2, bugrova, Itsmylove1
Желаем всем удачи и восхваляем солнце \[^-^]/
Upd. Разбор задач









Good luck everyone!
yo participated in this contest rated but i did not recive any elo? and when i go to my acounts contest all it shows codeforces round 1013 div 3 as unrated but i did rated whats the issue?
Check now.
It looks really cool. I'm considering participating in this contest, although I'll have to sacrifice some sleep due to the different time zones.
Hoping to get back to expert after a huge drop in rating -____-
Wow, what a coincidence to see you here again, I hope you can play superbly this time:)
As a tester, I hope you'll enjoy these problems
And praise the sun!
As a tester, I wish you good luck and Praise the Sun for your best participation!
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I hope that today I will become a specialist. Hope on this contest.
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Another cool round! \[^-^]/
Do you guys think this round is going to be harder than typical Div 3 rounds because it is based on an Olympiad?
Probably not. I assume (based on other such rounds) that, if the problems are too hard, some new, easier ones will be added, or some of the olympiad problems will be simplified
I hope your thoughts happen!
Good luck!
Hope! we have a contest where
I hope I can solve four problems in this round and finish A, B, and C within 30 minutes as I am a newbie.
Good Luck Everyone
Happy Coding
it would be cool if at least once every 2 weeks there would be a div 3 or div 4
good luck everyone!
its based on olympiad so its going to be interesting
I hope I can become Expert after this contest.
High five we have the same rating and the same goal!
Same
It's a chance to be back to CYAN...
The sooner you get it, the sooner you can solve it. Because, it's just a while loop template. Yes, I'm talking about BINARY SEARCH. Problems 'D' and 'E' — I have solved both using BINARY SEARCH! And again, I am going to be CYAN...
Good Luck Guys...
And praise the Sun !
Occasionally, it’s humorously linked to brute-force approaches (like iterating over all possible solutions) because brute-force can be seen as a last resort, much like praying for a solution. I hope you see something like this in the contest.
As a tester, I have a proof that upvoting this comment will lead to positive delta. But the proof is too long to fit the margin.
I downvoted, let's see what will happen! ಠ_ಠ
And, u aren't a tester T.T
what happens if i vote?
like div3.This means that I can see and solve more interesting problems instead of only doing three problems like div2
\^.^/
Great contest. Relax and solve as much problems as you can!
Having this kind of Laboratory and Olympiad event in your region is undoubtedly a blessing. From there, we get such tricky problems that AI models fail to generate solutions. So, bad luck for cheaters (who are going to downvote this!) and exciting for real CP lovers. And of course, I belong to the latter...
Hope for positive delta
Looking forward to touch 1000 after this contest. In fact, any amount of positive delta would motivate me.
GL HF! As a tester, I hope you will feel the maximum dope from the tasks and be able to achieve your best results!
Hopefully I will get my specialist back, kinda nervous tho as I feel it would be harder than normal div 3s since it's based on an olympiad
like div3.This means that I can see and solve more interesting problems instead of only doing three problems like div2
Hope the problems are not chatgpt able.
It's time to up expert:)
So can anyone tell me how to solve prob G?
UPD: Report a stupid Indian cheater's youtube, share the code from A to F during the contest
UPD : Apparently during system testing right now, I got TLE at test case 27 which is unfortunate plz ignore below.
Imagine you're Gleb, and you have to cross a river to reach a point exactly s meters away. Each time you paddle, your current power (or energy) determines how far you go. BUTTT every time you decide to turn around to adjust your course, your power drops by 1 (unless you're already at the minimum power of 1). So, you want to minimize turns because every turn costs you power.
I originally considered trying every possible sequence of moves forward and backward to see which one would land me exactly at point s. this was stupid i am stupid — charles leclerc
Now here's what I actually did:
Good ol dp. The key idea was to base my dp state on a modulus (p) and, for each residue modulo p, store a pair of numbers. These numbers represent the minimum and maximum effective move counts/strokes needed to reach that residue.
Now we can build the state transitions using modular arithmetic. For each residue in the current state, calculate the new residue after a move by taking into account the effect of the stroke. I solved a linear congruence using the extended euclidean algorithm to compute a modular inverse, which was crucial to adjust the equation based on the gcd. Now with rnges I determined the valid range of multipliers (stroke counts) that would keep the move within the acceptable bounds.
To showcas ethe actual movement, I defined two functions: one for forward moves (fwd) and one for backward moves (bwd). The forward function simulates strokes that add distance, while the backward function handles strokes that subtract distance (after a turn). Now if I alternate between these, I can build a detailed map of all the positions I could reach after any number of moves.
The final step was to check if the dp state allowed me to reach exactly point s. I did this by looking at the residue corresponding to s and verifying whether the range of move counts included a valid solution. Since every turn reduces my power, the whole process was aimed at finding a sequence of moves that minimizes the number of turns, thereby preserving as much of my initial power as possible.
Sorry if this is text heavy its hard to explain in text its why I prefer pen n paper
UPDATE : This is based on number theory and there's a better solution using bitset by Edu175 below mentioned as well.
Thank u very much!
I just used bitset 312451438
Thank u very much too, and I think this is much easier than exgcd
Cost a lot of time to code Miller-Rabin for problem E. Just to realize it's bad because n <= 10 ** 7.
Damn another minus delta for being such a noob.
I increased 1e5 to 1e6, thinking I increased 1e6 to 1e7 and got one penalty. Think about me, though. T_T
That was painful, but at least my man still have plus delta :(
Btw you do have time to attempt F is already awesome. I don't have time in my hands.
yeah that sucks
why to code miller-rabin when we can find primes in √n*log(log(n)) by sieve only.
Wasn't even think about the sieve till last 30 minutes of the contest, I got blind spotted.
that's really painful, sometimes happens with me also,stuck at one approach and debugging.
This round is amazing! I really enjoyed D and E.
was F fenwicktree + dp ??
prefixsum + dp is suffice.
Wow! Most balanced Div3 in recent time. Kudos to the everyone involved.
Praise the sun!
really good problems fun F
Can anybody tell me how can i use segment tree in this problem, in which i only have to get the sum of the answers in a range such that if value at that index is 1, then take it, or leave it. 312465645
you don't need segment tree bruhh , just prefix sums are needed.
Yes, I think prefix sum can do that. But I'm still unsure that how would i selectively get the sum in the range. For example, I'm adding +1 to all the indices in a range [l, r], even if some of them is'nt valid (c = '#'). Now, when retrieving the sum in this range, how would i make sure that sum of such indices are not considered, that's my problem.
set the dp value of # cells to 0 compute prefix sums on this dp and to get sum in range in same row u can move d to the left and d to the right at most in previous row u can can move sqrt(d^2 — 1) to the left or to the right cuz of euclidean dist
prefix sum on current and previous row
G wrong ans on test 7 :( could not debug, not sure if solution is correct though.
please review whoever submitted F in the last 30 minutes
I'm pretty sure 70% of them are fake
My position dropped from 600 to 1100 in last 30 min.. T_T
you're very fast today, will plus delta enough to reach expert?
Don't think so.. :]
why? :(
My screencast for this contest, for anyone interested: click
How do you come up the intuition for E? Any ideas and topics to practice ?
Tried to compute Sieve of Erastosthenes and then brute force it
Sieve of eratosthenes is a typical way to identify all the primes in some range of values. That's all that's really needed for the implementation part of things, and the idea is just knowledge of gcd and lcm defintions.
I was planning to try pushing the formula, but I simulated the example and discovered the pattern
F can be solved by assuming an edge between two points if it is reachable. After that, it is kind of dp on the graph which is pretty systematic.
Definitely TLE
I just cannot process the fact that I mindsolved $$$F$$$ 50 minutes ago of End of contest, and finished implementing it (along with debugging) , 10 minutes after end of contest , just realized my implementation skill is slow
What is your logic for F
it’s just a classic prefix sum dp problem
I've just realized that Igor can go to row
i - 1only from rowiI thought that he can go to row
i - 2from rowiif d is 2, but seems like this can't happennice problem btw
Oh no, i was solving different problem for 1 hour
The same thing happened to me, until I tried to solve the testcases using pen and paper, then I realized that I could go to the row right above me only :(
this made the problem easier, but I think it will be nice if there is a hard version using the idea that Igor can go to any row above him if the row is reachable ($$$\leq d$$$)
sorry
I am talking about problem F !^_^
What am i missing here for D ? 312462491 [](https://codeforces.me/contest/2091/submission/312473448)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ // this is the code
include <bits/stdc++.h>
using namespace std;
void solve(){
int n, m, k; cin>>n>>m>>k; int total = m * n; int value = (total - k) / n; if( value == 0) cout<<m<<endl; else{ cout<< m / (value + 1) << endl; }}
int main(){
ios::sync_with_stdio(0); cin.tie(0); int t; cin>>t; while(t--){ solve(); } return 0;} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
try long long?
For C, does anyone have a proof of why it is impossible to form such a permutation for even values of $$$n$$$?
in C when you have even n you cannot interchange 1 element at a time. What i mean to say is you need just 1 element to be at its own place for example 1 on a1 or 2 on a2, etc but when we have even n we swap 2 numbers or say we need to leave 2 numbers as is to its place for example -> 1 3 2 5 4 6 -> here 1 is left on one place but now we need to swap all other numbers say swapped 6 and 4 above and it becomes -> 1 3 2 5 6 4, but now you see the real problem any two consecutive numbers together because they will take their own place in some cycle which means -> 1 3 2 5 6 4 will become -> 4 1 3 2 5 6 and it is not allowed, i hope you were satisfied with my answer
Here's a mathematical explanation wrote it quickly in notion
Terrible statement for F. Understanding it took a lot of time. Request for the authors to take out checking of comprehension from CP.
E was relatively very easy , You guys should've removed the constraint of sum of n over all testcases doesnt exceed 1e7. Atleast then precomputation would come into play
hey, yea i agree it was very simple for an E. Removing the constraint would make it a proper E. (BTW even i am a huge fan of Kurt <3 )
is the intended solution of G is $$$O(k^3)$$$? I was frustrated when saw $$$O(k^3)$$$ solutions, because I thought that wouldn't pass TL
Hello, I want to inform you that the account bombardiro_crokodilo is mine and I accidentally sent two identical codes. Can you please ignore the code?
Can someone help to make this work 312418567
this got acc you don't have to check for i<MAXN because for any i that is larger than n you'll never find mi such that mi*i<=n
Thanks for explaining i missed that, going to have huge -ve delta ;(
there are many upcoming contests, i hope you get +ve delta next time
Hi, I participated in Round 1013 and registered as a contestant. I solved 4 problems during the contest, and my rating is below 1600. However, I was marked as "Unrated, Allowed". Could you please check if there was a mistake?
your submissions look sus
Because i am not real
Hi not real i'm real
On 8th April, we have another div-3 round scheduled. Obviously, it's a short interval between two div-3 rounds.
Who can explain O(1) solution for problem D? I seen ksun42's solution but can't understand it
First of all, we want to distribute the k desks as evenly across the n rows to keep the maximum number of desks in a single row as small as possible. The most desks we then get in a row is occupied = ceil(k/n). In such a row, we have free = m — occupied empty slots. Once more, we want to divide/break up the occupied slots in that row as evenly via the free slots: max_bench = ceil(occupied / (free+1)) (if you have x empty slots, you can break the desks in the row up into x+1 benches).
ksun42's solution uses the fact that ceil(x/y) = int((x+y-1)/y).
A little bit standard (-__-) but very fascinating. (*^_^*)
Why hasn't my rating got reflected, although I understand that i am not a trusted participant because I have participated in less than 5 contests, but it says if my rating was never above 1600 it would be considered as rated right, can someone explain.
You just not in offical standings table because you are untrusted. As you know, it is because of some reasons. not offical == unoffical
It doesn't means you are unrated. Just you are not in standings.
Your rating will be updated after we run with hacked submission's datas, same as others
Unless you checked as Unrated Participation...
problem B is exactly same as
https://codeforces.me/contest/1380/problem/C
My submissions just been ignored, they say that my solution for problem F is the same as of some others. However i didn’t sent my code or copy and paste a solution from anybody. I can’t prove it now but may be i could find proof later.