The 14 given integers lie in 14 cells of the above 4x7 array. As in the proof of
#26, let ai, i=1,...,7, be the number of pairs of rows of the array which each
contain a given integer in the i-th column of the array. Then the sum of the ai is
at least 7 since the minimum sum occurs when each column has two given integers.
Since the number of pairs of rows of the array is 4(4-1)/2=6<7, some 2 by 2 subarray
consists of given integers. Hence the sum of the two diagonal elements of this subarray equals the sum of its two off-diagonal elements.