HDUOJ 1001 Sum Problem

 1 #include <iostream>
 2
 3 using namespace std;
 4 long long sum(long long n);
 5
 6 int main()
 7 {
 8     long long n;
 9     while(cin >> n)
10         cout << sum(n) << endl << endl;
11     return 0;
12 }
13
14 long long sum(long long n)
15 {
16     return (1 + n) * n / 2;
17 }