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문제

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.

입력

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

출력

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

예제 입력 1

3

예제 출력 1

1

예제 입력 2

5

예제 출력 2

10

힌트

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