Addition of Integers

In the following demonstration of integer addition, the addends are modeled by using small blocks marked with 1 or -1. Press the gray buttons to form the first positive or negative addend and the green buttons to form the second addend.
Many thanks to Anthony OR 柯志明 for bringing the following article to my attention and for his valuable suggestions. Reference  Kaner, P.A. (1964) 'The addition and subtraction of negative numbers', The Mathematical Gazette,48(364), 183-185.