C++ If/else if statements

I am making a program (too much freetime) about cramer's rule and matrices. And somewhere, I have this problem where you will need to input "Yes" or "No". And If I typed other words (lol for example) it would not go to else if statement or ask the question again. How do I limit the answers to only Yes / No?

cout << "There! you have now your first equation! is it " << a << "x+"   << b << "y=" << e << "? Is it? Yes/No"<<endl;
cin >> z;
if ( z== "No" )
    {
        while ( z== "No" )
            {
                cout << "what is the value of x?"<<endl;
                cin >> a;
                cout << "\n"<<endl;
                cout << "what is the value of y?"<<endl;
                cin >> b;
                cout << "\n"<<endl;
                cout << "what is the value of answer?"<<endl;
                cin >> e;
                cout << "\n"<<endl;
                cout << "There! you have corrected your first equation! is it " << a << "x+" << b << "y=" << e << "? Is it? Yes/No"<<endl;
                cin >> z;
            }
    }
else if ( z== "Yes" )
    {
        cout << "We will now proceed!"<<endl;
    }

I am just a beginner in c++ using codeblocks