Actionscript 3 :: Constrain MovieClip Drag To A Circle?

I have a draggable slider that looks like this:

The blue bar has the instance name track and the pink dot has the instance name puck.

I need the puck to be constrained within the blue area at all times, and this is where my maths failings work against me! So far I have the puck moving along the x axis only like this:

private function init():void
{
zeroPoint = track.x + (track.width/2);
puck.x = zeroPoint-(puck.width/2);

[Code]....