when i use pdist2 function of Hamming distance between tow binary images the result is not percentage value

Dear Gentlemen i face problem when i use pdist2 function of hamming distance between two binary images in same size, the output result is an array not a percentage value, where i want to use hamming distance to calculate the percentage difference between two binary images:

Image1 = imread('S8.bmp');
Image2 = imread('S9.bmp');
level1=graythresh(Image1);
bw1=im2bw(Image1,level1);
level2=graythresh(Image2);
bw2=im2bw(Image2,level2);
D = pdist2(bw1,bw2,'hamming');

[John Chilleri]

Given array D, could you just say sum(D)/length(D) to obtain the desired percentage? Or sum(sum(D))/(size(D,1)*size(D,2))?