ÖMO 1986 Problem 2

For $s,t \in \mathbb{N}$ , consider the set $M=\{ (x,y) \in \mathbb{N} ^2 | 1 \le x \le s, 1 \le y \le t \}$ . Find the number of rhombi with the vertices in $M$ and the diagonals parallel to the coordinate axes.