Berkeley Math Circle Monthly Contest 4 #0hp7

Berkeley Math Circle Monthly Contest 4Hard

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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.

Problem

Problem:

The sequence of numbers $1, 2, 3, \ldots, 100$ is written on the blackboard. Between each two consecutive numbers a square box is drawn. Player $A$ starts the game and the players $A$ and $B$ alternate the moves. In each turn a player chooses an empty box and places "+" or "." sign in it. After all the boxes are filled the expression on the blackboard is evaluated and if the result is an odd number the winner is $A$ . Otherwise the winner is $B$ . Determine which of the players has a winning strategy and what the strategy is.