module Fibonacci;

import Stdlib.Prelude open;

fib : Nat → Nat → Nat → Nat
  | zero x1 _ := x1
  | (suc n) x1 x2 := fib n x2 (x1 + x2);

fibonacci (n : Nat) : Nat := fib n 0 1;

main : IO := readLn (stringToNat >> fibonacci >> printNatLn);