Question: How can I get a Probability Density Function for the distance between two points in a circle

I am trying to calculate a probability density function for the distance between two points inside a unit circle.  I have succeeded with doing this for a few differing applications, but I am stuck on this problem.  I use uniformly distributed polar coordinates to find x and y values for two independent points.  When I ask for the PDF of the distance, I get a FAIL.

I am assuming that such a calculation is possible, I've succeeded with similar problems, but here I am stuck.  I hope my problem is rooted in my lack of knowledge, as opposed to a limitation of Maple.

Here are my Maple statements:

with(Statistics);
th1 := RandomVariable(Uniform(0, 2*Pi));
th2 := RandomVariable(Uniform(0, 2*Pi));
r1 := RandomVariable(Uniform(0, 1));
r2 := RandomVariable(Uniform(0, 1));
x1 := r1*cos(th1);
y1 := r1*sin(th1);
x2 := r2*cos(th2);
y2 := r2*sin(th2);
Dist := sqrt((x1 - x2)^2 + (y1 - y2)^2);
f := simplify(PDF(Dist, t));

Does anyone have any idea as to what I am doing wrong?  Thank you.

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