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Berkeley Math Circle Monthly Contest 3 #0hox

Berkeley Math Circle Monthly Contest 3Hard

From module Strategy and Write-up Structure

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Understand the scale
Scale 0.5: Easiest math competition problems, often solvable even for early middle school students without experience. 1: Problems strictly for beginners, often at a moderate middle school level. 1.5: Problems for stronger beginner students, on the level of the middling problems in most middle school contests. 2: For motivated beginners, harder questions from the previous categories. 2.5: More advanced beginner problems, hardest questions from previous categories. 3: Early intermediate problems that require more creative thinking. 3.5: Tougher early intermediate problems that consistently stretch into higher-level creative thinking or conceptual knowledge. 4: Intermediate-level problems. 4.5: Upper intermediate problems approaching the upper bound for non-invitational sprint math competitions. 5: More difficult AIME problems or simple proof-based Olympiad-style problems. 6: High-level AIME-styled questions or introductory Olympiad-level questions. 7: Tougher Olympiad-level questions, may require more technical knowledge. 8: High-level Olympiad-level questions. 9: Expert Olympiad-level questions. 9.5: The hardest problems appearing on Olympiads which the strongest students could reasonably solve. 10: Historically notorious problems, generally unsuitable for even very hard competitions because they are exceedingly tedious, long, and difficult.
Coloring schemes, extremal argumentsInvariants / monovariantsGames / greedy algorithms

Problem

Problem:

Let n>3n>3 be an integer which is not divisible by 33. Two players AA and BB play the following game with an n×nn \times n chocolate table. First, player AA has to choose and remove one piece of the chocolate, without breaking other pieces. After his move, player BB tries to partition the remaining chocolate into 3×13 \times 1 (and 1×31 \times 3) rectangles. If BB manages to do so, then he/she is the winner. Otherwise the winner is AA. Determine which player has a winning strategy and describe the strategy.

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