I am writing a program (in cpp) to check the primality of a given number

The point where i am struck is , I need to check in between the program wether the value i obtained upon some arithmetic operations on the input is an integer or not

i.e lets say input is 'a'

I want to know how to check if 'b' is integer or not (FYI, b=(a+1)/6 )

My attempt for this :

int main()
{
    using std::cin;
    using std::cout;
    int b,c;
    int a;
    cout<<"enter the number";
    cin>>a;
    b=(a+1)/6;
    c=(a-1)/6;
    if (b is an integer)
        cout << "The given number is  prime";
    else if (c is an integer)
        cin << "The given number is  prime!";
    else
        cout<<"The number is not prime";                  
    return 0;
}