:- module fib2.
:- interface.
:- import_module io.

:- pred main(io::di, io::uo) is det.

:- implementation.
:- import_module int,list,string.

:- func fib(int) = int.

fib(N) = (if   N =< 2 
          then 1 
          else fib(N - 1) + fib(N - 2)).

main(!IO) :-
     io.read_line_as_string(Result, !IO),
     (if
         Result = ok(String),
         string.to_int(string.strip(String), N)
      then
         io.format("fib(%d) = %d\n", [i(N), i(fib(N))], !IO)
      else
          io.format("That isn’t a number...\n", [], !IO)
      ).