#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using pii = pair<int, int>;

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  
  int n;
  while (cin >> n) {
    cout << n * (n + 1) / 2 + 1 << endl;
  }

  return 0;
}
