func fib(n)
  if 1 >= n then return n; end;
  var fib_n := 1; (* fib(n) *)
  var fib_n1 := 0; (* fib(n-1) *)
  var r := 0;

  while n >= 2 do
    (* calculate fib(n+1) *)
    r := fib_n + fib_n1;
    (* update fib(n-1) and fib(n) *)
    fib_n1 := fib_n;
    fib_n := r;
    n := n + (-1);
  end;
  return r;
end;