use Carp qw/croak/;
sub isPrime (_)
{
	my $num = shift;
	
	return if $num == 1;  # 1 is not prime
	
	croak "usage: isPrime(NATURAL NUMBER)"
		unless $num =~ /^[1-9][0-9]*$/;
	
	for my $div (2 .. sqrt $num)
	{
		return if $num % $div == 0;
	}
	return 1;
}