Finding energy in matlab

Energy can be found based on the matlab documentation using:

stats = graycoprops(glcm, properties)

For example, I typed this:

>> a = [1 2; 3 4]

a =

       1     2
       3     4

>> stats = graycoprops(a, {'energy'})

stats = 

      Energy: 0.3000

Here, I want to ask. If we want to do this manually, how did we get the Energy value shown above. This is the part I didn't get clearly.

Thanks.