#!/usr/bin/perl -w
use strict;

sub fibonacci {
    my $fib = shift;
    return $fib if ! $fib || $fib == 1;    
    my ($low, $high) = (1, 0);
    for (1 .. $fib) {
        ($low, $high) = ($high, $low + $high);
    }
    return $high;
}

print fibonacci(20), $/;