Calculator App (RPN)

So I am working for the interface of an RPN calculator, Reverse Polish Notation, if dont know what is wikipedia is your friend :). The algorithm itself simplify a lot the userinterface. I liked to use here big button that made it look very easy in my opinion. Tell me what you think guys. Here you can find a better image http://cl.ly/0z3r3B2v0q161P3k430j