20. For $n \in N$, let $S(n)$ denote the number of distinct ordered pairs $(x, y)$ of integers such that $x \geqslant 0, y \geqslant 0, 2x + y \leqslant n$. Let $n, m$ be natural numbers with $n > m$. Then express $S(n+m) - S(n-m) =$ $\qquad$
20. $m(n+2)$