flash actionscript: Hi,

Im tryin to scroll some ground.

I have got it scrolling from top to bottom, but would like to scrolli the
other way - top to bottom

Here is my code:

onClipEvent (load) {
road.duplicateMovieClip("ground2", 00);
ground2._y = road._y+road._height -1;
groundStarty = this._y;
groundSpeed=20;
}
onClipEvent (enterFrame) {
this._y-=groundSpeed;
if (this._y<= (groundStarty-road._height)){
this._y=groundStarty-groundSpeed;
}
}

Hope someon can help

Thanks

Tried -20 but it only scrolls a bit then dis apperars