Question 23
Square $ABCD$ is divided into $n^2$ equal elementary squares, by drawing parallel lines to their sides. We call the vertices of the elementary squares points of the lattice. One rhombus will be called “good”, if it is not a square, their vertices are points of the lattice and their diagonals are parallel to the sides of the square $ABCD$. Find a closed formula, with respect to $n$, of the number of “good” rhombuses, where $n$ is integer greater than $2$.