probably very easy workflow question?

hello, I'm completely new to After Effects, have worked through several tutorials but i have a workflow question.

I have a numbered series of tiffs. i want to put this series in my timeline, following a reversed series of the same tiffs. so, i want frame 400-1, followed by 1-400 (equaling 800 frames), using the same sequence of images.

What is the simplest way to accomplish this?

thanks,

NJ