(* Tail recursive version *)
let fib n =
  let rec fib_aux (n, a, b) =
    match (n, a, b) with
      | (0, a, b) -> a
      | _ -> fib_aux (n-1, a+b, a)
  in
    fib_aux (n, 0, 1)
      
let _ =
  if Array.length Sys.argv <> 2 then
    begin
      Printf.printf "Usage: %s number\n" Sys.argv.(0);
      exit 1
    end
  else
    begin
      print_int (fib (int_of_string Sys.argv.(1)));
      print_newline ()
    end