1001 Sum Problem

#include<iostream>
#include<algorithm>
using namespace std;
int main()
{
    long long a;
    while(cin>> a){
        if(a%2 == 0) cout << a/2 * (a+1) << "\n\n";
        else cout << (a+1)/2 *a << "\n\n";
    }
    return 0;
}

这种题考的就是范围