#include<iostream>
#include<iomanip>
#include<vector>
#include<queue>
#include<list>
#include<stack>
#include<algorithm>
#include<cmath>
#include<memory>

using namespace std;

int main()
{
  int n;

  while ( cin >> n )
    {
      cout << n * (n + 1) / 2 + 1 << endl;
    }
  
  return 0;
}