factorial

use v5.18;

# http://math-prosto.ru/?page=pages/factorial/factorial.php

sub fac {
    my ($n) = @_;
    return 1 if $n == 0 || $n == 1;
    my $res = 1;
    for (my $i = 1; $i <= $n; $i++) {
        $res *= $i;
    }
    return $res;
}

say fac 0;
say fac 1;
say fac 5;