Calculate A+B

Little Johnny walked into the kitchen, saw his mother making a cake and announced, "I'll be playing in my room for the next two hours. I sure would like a piece of cake when you're finished."
Later, when his mother brought him a piece of cooled cake, Little Johnny exclaimed, "Golly, it worked!"
Puzzled, his mother asked, "What do you mean?"
Little Johnny replied, "Daddy said that in order to get a piece around here, you have to spend a couple of hours playing first!"