Alpha on drag

hey, I am such na noob, this on is killing me. I’ve created two movie clips, each in every layer.
Top one is dragable to the left (only horizontal drag) so when I drag it to the left I can just see the bottom one. Simple as that
I’ve used following code for draging the top one

top.onPress = function() {
	startDrag(this, false, 576, 450, -3000, 450);
top.onRelease = top.onReleaseOutside=function () { this.stopDrag();};

The thing is that I want to add a alpha effect while draging. So as a start draging the top mc’s alpha should be 100%, and than as I drag it alpha should change (to the 0% in the final position (most to the left position))

Any suggestions