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.

1. Input

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

2. Output

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

3. Sample Input

{{{-3 }}}

4. Sample Output

{{{-5 }}}

   1 #include <iostream>
   2 using namespace std;
   3 
   4 int main()
   5 {
   6   int n;
   7   cin >> n;
   8   if(n>0) {
   9     cout << n*(n+1)/2 << endl;
  10   } else {
  11     cout << n*(-n+1)/2+1 << endl;
  12   }
  13 }