
//Nayeem Shahriar Joy ,Applied Physics And Electronic Engineering , University of Rajshahi.

  #include <iostream>
  #include <cstdio>
  #include <algorithm>
  #include <cstring>

  #define MAX 105000

  using namespace std;

  int main()

  {
      int n;

      while(cin>>n)

      {
          cout<<((n*(n+1))/2)+1<<endl;
      }

      return 0;
  }