Polygon with N-Sides

You have given an integer $N$. A Regular Polygon is an $N-sided$ Polygon in which all the sides are of the same length and are symmetrically placed about a common center (i.e. the polygon is both equiangular and equilateral). Examples- Equilateral Triangle, Square, Pentagon, Hexagon, Heptagon, Octagon and so on. Your task is to find the Number of Lines that can be formed inside the Regular $N-s