HDUOJ 1001: Sum Problem

#include <iostream>

using namespace std;
long long sum(long long n);

int main()
{
    long long n;
    while(cin >> n)
        cout << sum(n) << endl << endl;
    return 0;
}

long long sum(long long n)
{
    return (1 + n) * n / 2;
}