C program to check whether a given number is prime or not

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

void main()
{
int num, j, flag;

clrscr();

printf("Enter a number\n");
scanf("%d", &num);

  if ( num <= 1)
{
      printf("%d is not a prime numbers\n", num);
  exit(1);
    }

flag = 0;

    for ( j=2; j<= num/2; j++)
{
if( ( num % j ) == 0)
{
  flag = 1;
break;
}
}

      if(flag == 0)
printf("%d is a prime number\n",num);
      else
printf("%d is not a prime number\n", num);
}
/*------------------------
Output
RUN 1
Enter a number
34
34 is not a prime number

RUN 2
Enter a number
29
29 is a prime number
-----------------------------*/