olymmath

ELMO SL 2019 C3

    In the game of Ring Mafia, there are 20192019 counters arranged in a circle. 673673 of these counters are mafia, and the remaining 13461346 counters are town. Two players, Tony and Madeline, take turns with Tony going first. Tony does not know which counters are mafia but Madeline does.

    On Tony's turn, he selects any subset of the counters (possibly the empty set) and removes all counters in that set. On Madeline's turn, she selects a town counter which is adjacent to a mafia counter and removes it. Whenever counters are removed, the remaining counters are brought closer together without changing their order so that they still form a circle. The game ends when either all mafia counters have been removed, or all town counters have been removed.

    Is there a strategy for Tony that guarantees, no matter where the mafia counters are placed and what Madeline does, that at least one town counter remains at the end of the game?

    Proposed by Andrew Gu

    Dataset text AI-MO AoPS. Quoted with source.

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