How I can find Mean Absolute Deviation for image by matlab

If I have image with this dimension 240x180 and I want to use Matlab to find Mean Absolute Deviation (MAD) for each sub-image ( 20x20) from the original image so I must extract 108 results at the end , I know the concepts of MAD by finding mean for each 20x20 sub-pixel then find the summation of the absolute value of the difference between each pixel and calculated mean. I don't know how I can start to do that in Matlab and what is the steps I should follow it to arrive to this result and what is the best tool box I can use it in this case!