Editorial for COCI '07 Contest 1 #1 Cetvrta

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We start by observing four points $(x_1, y_1)$ ~(x_1, y_1)~, $(x_2, y_2)$ ~(x_2, y_2)~, $(x_3, y_3)$ ~(x_3, y_3)~, $(x_4, y_4)$ ~(x_4, y_4)~ which form a rectangle. Numbers from the set $\{x_1, x_2, x_3, x_4\}$ ~\{x_1, x_2, x_3, x_4\}~ take two distinct values (each value two times). If we remove any point, its x coordinate can be determined by observing which value appears only once. The same holds for the y coordinate.

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