module Fibonacci;

open import Stdlib.Prelude;

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

fibonacci : Nat → Nat;
fibonacci n := fib n 0 1;

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