#include <stdio.h>
#include <stdbool.h>
#include <inttypes.h>
#include <math.h>

bool is_prime(uint64_t n) {
	if (n < 2) return false;
	if (n < 4) return true;
	if (n % 2 == 0 || n % 3 == 0) return false;
	uint64_t sqrt_n = sqrt(n);
	for (uint64_t i = 5; i <= sqrt_n; i += 6) {
		if (n % i == 0 || n % (i + 2) == 0) {
			return false;
		}
	}
	return true;
}

int main(int argc, char **argv) {
	if (argc < 2) {
		fputs("Pass a positive integer as an argument.\n", stderr);
		return 1;
	}
	uint64_t input;
	if (argv[1][0] == '-' || sscanf(argv[1], "%" SCNu64, &input) != 1) {
		fputs("This is not a positive integer.\n", stderr);
		return 1;
	}
	printf("This integer %s prime.\n", is_prime(input) ? "is" : "is not");
	return 0;
}