KöMaL 2019 C1571

The positive integers from 1 to $n^2$ are written in increasing order in an $n\times n$ table: the numbers 1 to $n$ are entered in the first row, ($n+1$) to $2n$ in the second row; and so on. Prove that the sum of the numbers in one diagonal equals the sum of the numbers in the other diagonal.