#include <bits/stdc++.h>
using namespace std;
/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main(int argc, char** argv) {
int n,a[501]={1};
for(int k=1; k<=500; ++k) a[k] = a[k-1]+(k-1);
while(cin>>n)
{
cout << a[n] << endl;
}
return 0;
}