19810번 - Isosceles triangles 다국어

You are given a regular polygon with $n$ vertices. You should find the number of isosceles triangles with vertices that are the vertices of the polygon.

The only line of input contains one integer $n$ --- the number of vertices of a regular polygon ($3 \le n \le 10^9$).

The output line should contain a single integer --- the number of isosceles triangles with vertices that are the vertices of the polygon.

An isosceles triangle is a triangle that has at least two equal sides.