int is_prime(long num)
{
	int i = 5;
	int w = 2;

	if (num == 1)
		return 0;
	if (num == 2 || num == 3)
		return 1;
	if (!(num%2) || !(num%3))
		return 0;

	while ((i*i) <= num) {
		if (!(num%i))
			return 0;
		i += w;
		w = 6 - w;
	}

	return 1;
}