timus1068

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Sum

http://acm.timus.ru/problem.aspx?space=1&num=1068

Time Limit: 2.0 second

Memory Limit: 1 000 K

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 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

Sample Output

-5

---

#include <iostream>
using namespace std;

int main()
{
  int n;
  cin >> n;
  if(n>0) {
    cout << n*(n+1)/2 << endl;
  } else {
    cout << n*(-n+1)/2+1 << endl;
  }
}