Observing Frogs

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Points: 5 (partial)
Time limit: 0.1s
Java 0.5s
Memory limit: 16M

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Problem type

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

4

Sample Output

8

Sample Input 2

5

Sample Output 2

12

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