Calculator

I tried to make a calculator UI for everyday use in peace. Anyway, I usually think that It's difficult to know how to work "C" especially without "AC" like a iPhone's calculator UI. So, I decided to use "AC" and "UNDO". I put "UNDO" to the bottom since it is often used. And not to press by mistake, I used different color for it. Moreover, the color is dark to show it works negative.