How to identify a number on the screen with Bitmap?

Question

I am new in C# and I plan to make a program to identify numbers on the screen and turns them or saves them into a Double variable. The only thing I could do was take the screen print :P

 Bitmap printtela ()
    {

        Bitmap novaleitura = new Bitmap(Screen.PrimaryScreen.Bounds.Width, 
        Screen.PrimaryScreen.Bounds.Height);
        Graphics g = Graphics.FromImage(novaleitura);
        g.CopyFromScreen(0, 0, 0, 0, Screen.PrimaryScreen.Bounds.Size);

        return novaleitura;
    }

I searched Youtube and tried to access the code of this video: https://www.youtube.com/watch?v=gEgxZrXPnzc&t=172s However, even the copy of the video code did not run correctly :P