Write a C++ program to determine whether a given number is prime or not

#include <iostream>
using namespace std;
void main()
{
  int num,i,ctr=0;
    cout<<"Input  a number: ";
    cin>>num;
    for(i=2;i<=num/2;i++){
        if(num % i==0){
         ctr++;
            break;
        }
    }
   if(ctr==0 && num!= 1)
        cout<<num<<" is a prime number.\n";
   else
      cout<<num<<" is not a prime number";
      system ("pause");
}

Test Data : 

Input a number: 13 

Expected Output :

13 is a prime number.