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1068. Sum

Time limit: 2.0 second
Memory limit: 64 MB
Your task is to find the sum of all integer numbers lying between 1 and N inclusive.

Input

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

Output

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

Sample

inputoutput
-3
-5
Problem Source: 2000-2001 ACM Northeastern European Regional Programming Contest (test tour)