![]() But I had also, at least, come up with the relation between $$$E_x$$$ and $$$E'_x$$$, and I thought this would solve the problem, before reading the tutorial. Indeed it is hard to determine the proper approach to calculate the answer in this problem, I had realized that before the contest. It's a nice problemset and a nice problem, which can be said about a lot of superhard AGC Fs. ![]() There's a lot of such things and it's actually quite common that a problem has a simple solution, but it's super hard because of many other non-solutions. It's possible to miss the realisation that it's actually simple. How to compute the constant $$$C$$$ in the editorial? The expected time of reaching "$$$i$$$ has everything" from "$$$j$$$ has everything" without reaching "$$$i$$$ has everything" before can stop someone because it looks like it has states (number of tokens of $$$i$$$, number of tokens of $$$j$$$) which is $$$O(sum^2)$$$. What sort of math is expected, how about some matrix algebra? (When it looks like simpler things don't work, you try other, more complex things.) How about separating cases where some token doesn't move (which makes the person who wins obvious) and where it does move? There's a lot of paths you can try to take towards a solution. When should we sum over $$$i$$$ and what should we obtain from it? How about modifying the problem by allowing giving a token back to the same person? It leads to some more symmetry, after all. What if you don't see that it leads to a solution? You might abandon the right solution and try something else that seems more doable. Here's the first question: is the expected value of this simpler number of steps finite? Such details are pretty common and need you to basically proceed in a direction with confidence that it's the right direction. An experienced contestant can quickly notice both that the solution can focus on the final person and that it can be expressed as the simpler "reach an end state without stopping at the first end state" minus a correction term. The idea from the editorial is very similar to what I started with. ![]() It's that there are so many paths that seem viable but require working with equations on paper with no guarantee that what you're doing will lead to a fast enough solution. It's not that it requires some hardcore trick or theory. Ok, after solving it, I can see where the problem lies. Hope you have a nice day! Also you can view a blog by our tester Hazyknight about his opinions of this round: We are sorry about our mistakes, and hope you will like these problems after reading editorials here: And also, in some problems pretests are weak. UPD: Hey, it seems that Div.1 is really hard and has bad discrimination. Wish all of you good luck and have fun! Since the round is rated, we also wish you guys have huge positive $$$\Delta$$$ in this round! We have made an effort to create interesting problems, strong tests and clear statements. MikeMirzayanov, for excellent Codeforces and Polygon platform. Glamorgan, for proofreading and polishing our statements Slime, for giving suggestions and support throughout the whole preparation of this round WZYYN, Elegia, skip2004, vintage_Vlad_Makeev, fpdqwq, wangziji, Suika_predator, wrg0ababd, AcF-_-FcA, Shuba_Buba, dysyn1314, xiaolou0411, LiM_256, Hazyknight, Sad_reacts_only, djq_cpp, little_waxberry, for their hard-working testing and suggestions We would like to express our sincere gratitude to:ģ00iq, for responsible and interesting coordination ![]() Problems of this round were prepared by Rebelz, A.K.E.E., mydiplomacy and me BlueSmoke. In both divisions, you will have 2.5 hours to solve 6 problems. We are excited to invite you to Codeforces Round #641 (Div.
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