UI Challenge 004 Calculator

Hello, this is my first work here. I'm starting to build my knowlegde and experience, after all of the tutorials, it's time to make some my own work :) I know this could be much much better, but I am satisfied as it is my first work and participation in UI Challenge! I hope some of you will like it, and that those who don't will give some constructive feedback. I'm glad I have joined such a big and beautiful community! Cheers!