SJECIŠTA

문제

Consider a convex polygon with N vertices, with the additional property that no three diagonals intersect in a single point. Find the number of intersections between pairs of diagonals in such a polygon. 

The figure below shows one such polygon with 6 vertices. 

Note: a polygon is convex if all of its interior angles are less than 180 degrees.

입력

The first and only line of input contains a single integer N, 3 ≤ N ≤ 100. 

 

출력

Output the number of intersections on a single line. 

 

3

0

4

1

6

15