let id x = x

let rec num_of_area n = 
  if n = 1 then 2
  else n + num_of_area (n - 1)
                       
let _ =
  try
    while true do
      let x = Scanf.scanf "%d\n" id in
      Printf.printf "%d\n" (num_of_area x)
    done
  with
    End_of_file -> ()