The numbers from 1 to 64 are written on the squares of a chessboard (from 1 to 8 from left to right on the first row , from 9 to 16 from left to right on the second row , and so on). Pluses are written before some of the numbers, and minuses are written before the remaining numbers in such a way that there are 4 pluses and 4 minuses in each row and in each column . Prove that the sum of the written numbers is equal to zero. combinatoricsChessboardSum