#include<stdio.h>
#include<math.h>
void main(void)
{ 
	int n,i,k,r;
	printf("input n: ");
	scanf("%d",&n);
	if(n==2)
		printf("2 is a prime");
	else if(n>2)
	{ 
		i=1;
		k=sqrt(n);
		do
		{ 
			++i;
			r=n%i;
		}while(r&&i<=k);
		if(r)
			printf("%d is a prime\n",n);
		else
			printf("%d isn't a prime\n",n);
	}
}