#include<iostream>

using namespace std;

//  A_n - A_(n-1) = n 
main(){
  int n;
  
  while(cin >> n){
    int out =2;
    
    for(int j=2;j<=n;j++){
      
      out += j;
      
    }
    cout << out << endl;
  }
  
  return 0;
}