fib.pas

program Fib;

// Allows for `Result` instead of the function name.
{$mode ObjFPC}
// Disables "IO checking" for `WriteLn`.
{$I-}

function Fib(const N: UInt64): UInt64; inline;
begin
  case N of
    0, 1: Result := N;
    otherwise Result := Fib(N - 1) + Fib(N - 2);
  end;
end;

begin
  WriteLn(Fib(47));
end.