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Problem D
Sjecista
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.
![\includegraphics[width=5cm]{hexagon.png}](/problems/sjecista/file/statement/en/img-0001.png)
Note that a polygon is convex if all of its interior angles are less than 180 degrees.
Input
The first and only line of input contains a single integer $N, 3\leq N\leq 100$ denoting the number of vertices.
Output
Output the number of intersections on a single line.
| Sample Input 1 | Sample Output 1 |
|---|---|
3 |
0 |
| Sample Input 2 | Sample Output 2 |
|---|---|
4 |
1 |
| Sample Input 3 | Sample Output 3 |
|---|---|
6 |
15 |