#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

void usage(char *progname, char *errmsg)
{
	if (errmsg)
		fprintf(stderr, "ERROR: %s\n", errmsg);

	fprintf(stderr, "Usage: %s <number>\n", progname);
	exit(1);
}

bool isprime(int number)
{
	for (int i=number-1; i>1; i--) {
		if ((number % i) == 0) {
			printf("%d is even divisable by %d\n", number, i);
			return false;
		}
	}

	return true;
}

int main(int argc, char* argv[])
{
	int number;

	if (argc != 2)
		usage(argv[0], "missing argument");

	number = atoi(argv[1]);

	if (isprime(number)) {
		printf("%d is a prime number.\n", number);
	} else {
		printf("%d is NOT a prime number.\n", number);
	}

	return 0;
}