#include <bits/stdc++.h>
int main() {
  int a, i, s = 0, r, m;
  scanf("%d", &a);
  r = (a * (a + 1)) / 2;
  m = a - 2;
  for (i = 1; i <= a - 2; (i++) && m--) {
    s = s + m * i;
  }
  printf("%d", r + s);
  return 0;
}
