HDOJ 1001_Sum Problem

AC代码：

#include <iostream>
#include <cstdio>
using namespace std;
int main(void)
{
    freopen("in.txt","r",stdin);
    long long n;
    while(scanf("%lld",&n)!=EOF)
    {
        if(n%2==0)
            printf("%lld\n\n",n/2*(n+1));
        else
            printf("%lld\n\n",(n+1)/2*n);
     } 
    
    fclose(stdin);
    
    return 0;
}