#include<iostream>
using namespace std;

int main(){

  long long int a,n;

  while(cin >> n){

    for(int i=1;i<=n;i++){
      if(i == 1) a = 2;
      else a += i;
    }

    if(n == 0) cout << 0 << endl;
    else cout << a << endl;
  }
}