Go4Expert - problem in calculating size of derived classes

Code:

class base

   { 

    private :

                int b;

   };

class derived : public base

    {

     private :

                int j;

    };

void main()

{

cout<<endl<<sizeof(derived)<<endl<<sizeof(base);

}

Output:
4
2
I think size of derived=sizeof(j)+sizeof(b).
Am I right or wrong?If I am wrong then please tell me how the size of derived class is
calculated.If I am right then tell me
Why the sizeof derived class is 4 when it is not inheriting the private member b of class base?