[Ffmpeg-user] reading a series backwards

Hello, I have had really good results so far with FFMPEG, so thanks to all the developers! I don't know how I can accomplish my latest task, so any help would be appreciated. I want to convert a two series of gifs into a mpeg. Both series run from 0 to 100, but I also want each series to run backwards from 100 to 0. Therefore, the complete mpeg will contain the four series as given below: 0 -> 100 first series of gifs 100 -> 0 first series 0 -> 100 second series 100 -> 0 second series

From the documentation, I can include multiple inputs with multiple '-i', but is there a way to read the series backwards?

Thanks, Brent