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