2000-2001 ACM Northeastern European Regional Programming Contest

Problem Z

"Sum"

Input fileSUM.IN
Output fileSUM.OUT
Time limit15 seconds per test

Your task is to find the sum of all integer numbers lying between 1 and N inclusive.

Input

The input file consists of a single integer N that is not greater than 10000 by it's absolute value.

Output

Write to the output file a single integer number that is the sum of all integer numbers lying between 1 and N inclusive.

Sample input

-3

Output for the sample input

-5