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Turkey Junior Olympiad 2006, Part II - P3

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January 20, 2013

Problem Statement

In the beginnig, all nine squares of 3×33\times 3 chessboard contain 00. At each step, we choose two squares sharing a common edge, then we add 11 to them or −1-1 to them. Show that it is not possible to make all squares 22, after a finite number of steps.
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