Prime number or not in C

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
main()
{
int num,j,flag;
clrscr();
printf("Enter a number:\n");
scanf("%d",&num);
if(num<=1)
{
printf("%d is not a prime number\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);
}
getch();
return 0;
}