[math-fun] help needed with a definite integral

Can anyone help with following integral, which arises in computing the average distance between two random points in a unit square? (Maple and Mathematica can't do it, at least the versions that I have.) Let h = 1/2, to save space. Int_{x = 0..h} Int_{r=-h-x..h-x} Int_{y=0..h} Int_{s=-h-y..h-y} sqrt(r^2+s^2) dx dr dy ds A numerical value would be fine - and a method - there are several other integrals that are of interest. Thanks! Neil