vector<bool> getSieve(unsigned int n) {
	vector<bool> sieve(n + 1, true);
	sieve[0] = sieve[1] = false; // 0 and 1 are not prime
	unsigned int i = 2;
	while (i <= (unsigned int) ceil(sqrt(n))) {
		if (sieve[i]) {
			for (unsigned int j = i * 2; j < sieve.size(); j += i) {
				sieve[j] = false;
			}
		}
		++i;
	}
	return sieve;
}