Lyestria is observing a group of ~N~ frogs. The frogs are numbered from ~1~ to ~N~.

The frog numbered ~i~ conveniently lives at the point ~i~. On this day, the frog numbered ~i~ wants to go to the point ~N-i+1~, where all the points are equally spaced.

Lyestria then asks you to compute the sum of the distances each frog must travel to get to their desired location.

Constraints

~1 \le N \le 10^9~

Input Specification

The first line contains one integer, ~N~, the number of frogs.

Output Specification

On one line, output one integer, the sum of the distances each frog must travel to get home.

Sample Input 1

4

Sample Output 1

8

Sample Input 2

5

Sample Output 2

12