Calculate number of pixels changed by subtracting two BW images

Hello, I need to calculate number of pixels whose color changes in two BW images consecutively taken with my camera. Can someone provide me the code? Thanks!

For instance I have 1.jpg and 2.jpg.

if true
a = imread('1.jpg');
b = imread('2.jpg');
Z = imsubtract(a,b);
  % code
end

After this code, I think I have to count the number of non-zero values in 'Z'?

I want to include the pixels changing either way (i.e. from black to white, and from white to black).

Thanks! Yang