#include <iostream>

using namespace std;

unsigned long long int hoge(int n) {
  static unsigned long long int data[10001] = {};
  if (data[n])
    return data[n];
  return data[n] = (n == 1) ? 2 : hoge(n-1) + n;
}

int main() {
  int n;
  while (cin >> n) {
    cout << hoge(n) << endl;
  }
}
