Adjusting individual slide timing in MP4 from PowerPoint

I converted a 20 slide ppt into a MP4 at 10 seconds per slide (this was recommended because there's animation on many of the slides). I used 10 seconds because that is the shortest a slide will end up being (slides 1 and 20). All others will need to be longer and, in many cases, much longer - ranging from 12 seconds to over 4 minutes for the longest one. I estimate the entire video will run ~25 min. Before I put music to it all, I think I need to break up the 20 slides into 20 pictures or clips? I tried following online instructions, but no luck. This is where I am now stuck. Any help would be greatly appreciated. Thanks