#include <iostream>

using namespace std;

/** Application main entry point. */
int main (
  int     argc,
  char  * argv[ ]
  )
{
  while ( true )
  {
    int n;

    if ( !( cin >> n ) )  break;
    cout << ( 1 + n * ( n + 1 ) / 2 ) << endl;
  }

  return ( 0 );
}