#include<bits/stdc++.h>
using namespace std;
int main(){
  int n,a=1;
  cin >> n;
  for(int i=1; i<=n; i++){
	a+=i;
  }
  cout << a << endl;
  //ans = 1 + i = 2
  //ans = 2 + i+1 = 4
  //ans = 3 + i+2 = 7
 return 0;
}
