use strict;
use warnings;

sub multiply_integers
{
	die "two positive integers!\n"
		if @_ != 2
		|| grep { not /^[1-9][0-9]*$/ } @_;
	
	my ($x, $y) = @_;
	return $x if $y == 1;
	return $x + multiply_integers($x, $y-1);
}

print multiply_integers(7, 6), "\n";