HDOJ 1001 Sum Problem

Problem Description

Hey, welcome to HDOJ(Hangzhou Dianzi University Online Judge).
In this problem, your task is to calculate SUM(n) = 1 + 2 + 3 + ... + n.

Input

The input will consist of a series of integers n, one integer per line.

Output

For each case, output SUM(n) in one line, followed by a blank line. You may assume the result will be in the range of 32-bit signed integer.

Sample Input

1
100

Sample Output

1
5050

#include 
int main()
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        int ans = 0;
        if(n%2==0)
            ans=n/2*(n-1)+n;
        else
            ans=(n-1)/2*n+n ;
        printf("%d\n\n",ans);
    }
}