Maximum sum

You are given a 2D matrix $mtr$ with $n$ rows and $m$ columns containing only integers. You can perform an operation on this matrix any number of times (possibly 0), in each operation you have to perform the following two steps. - Choose any two elements $mtr[i_1][j_1]$ and $mtr[i_2][j_2]$ such that $|i_1-i_2|\leq 1$, $|j_1-j_2|\leq 1$ and $|i_1-i_2|+|j_1-j_2|\geq 1$. - Set $mtr[i_1][j_1] = -mtr[i