Complete Prime Checker Function

/*
Write a C program to use a function int checkPrime(int n)
that receives one number and returns 1 if number
is prime or zero if number is composite. It will return 2
if number is 1, because 1 is neither prime nor
composite, it will return 3 if 0 or negative number
is entered by user
*/
int checkPrime(int n); /*function prototype or declaration*/
#include<stdio.h>
int main()
{
   int num1,ans;
   /* Enter one number in main() */
   printf("Enter a number for prime checker program=");
   scanf("%d", &num1);
   /* call the function by sending one number */
   ans = checkPrime(num1);
   if ( ans ==1)
     printf("%d is a Prime number",num1);
   else if ( ans==0)
     printf("%d is a Composite number",num1);
    else if(ans==2) 
    {
	 printf("1 is neither prime nor composite \nSince by definition every prime has two divisors");
	 printf("\nbut 1 has only one divisor that is 1\n And one is not composite \n");
	 printf("because every composite has more than two\ndivisors but 1 has only one divisor");
    }
	else 
      printf("Invalid input, enter only positive natural number");
   return 0;  
}
/* wrtite down function definition */

int checkPrime(int n)
{
	int i, flag=1; /*flag=1 means number is prime*/
	
	if(n==1)
	  return 2; /* 1 is neither prime nor composit*/
	else if (n<=0)
      return 3; /*invalid input*/
	  	
	for(i=2;i<=(n/2);i++)
	if(n % i == 0)
	  {
	    flag=0;
	    break;
	  }
	  if(flag==1)
	   return 1; /*flag=1 means number is prime*/
	   else
	   return 0; /*number is composite*/
}