Stopping my movie clip at _x = o

1

I have made a modified infinate menu, and I would like to make it stop at the begining and the end.

Here is the code that I have so far. What can I add to it to make it happen?

onClipEvent (load)
{
speed=-1;
}

onClipEvent (enterFrame)
{
_x+=(speed);
if (_x > 0) _x=-4603.3;
if (_x < -4603.3) _x=0;
}