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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