Matlab: zoom out by block

%zoom out by block
clear all;
close all;

aa=imread('r.jpg');
a=rgb2gray(aa);

c=size(a);

k=1;%height of new image
l=1;%width of new image


[height,width,color]=size(a);


sum=0;

for m =1:2:c(1)    %increase by 2
    for n =1:2:c(2)%increase by 2
         sum=sum+a(m,n);
         sum=sum+a(m,n+1);
         sum=sum+a(m+1,n);
         sum=sum+a(m+1,n+1);
        
                b(k,l)=sum/4;
         
         sum=0;
        
        % n=n+2;
        
         l=l+1;
        
         if(n>=width-2)
             break;
         end
    end
   
    k=k+1;
    l=1;
   
    n=1;
  %  m=m+2;
   
    if(m>=height-2)
        break;
    end
end

figure()
imshow(a)
figure()
imshow(b)
imwrite(b,'b.jpg');