#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) for (int i = 0; i < (n); i++)

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