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INMO 2021 Problem 4

    A Magician and a Detective play a game. The Magician lays down cards numbered from 11 to 5252 face-down on a table. On each move, the Detective can point to two cards and inquire if the numbers on them are consecutive. The Magician replies truthfully. After a finite number of moves, the Detective points to two cards. She wins if the numbers on these two cards are consecutive, and loses otherwise.

    Prove that the Detective can guarantee a win if and only if she is allowed to ask at least 5050 questions.

    Proposed by Anant Mudgal

    Dataset text AI-MO AoPS. Quoted with source.

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