Summary Report - Reverse Running Total?

Hi everyone! I'm currently working on a summary report and need help with creating a "reverse running total"(?)

Here is is an image of what I have in my app now:

Here is what I would like to get it to:

For the field on the right of "Number of All pets", I would like to take the total of all my pets and subtract them from the "Number of All pets" to the left. Going downwards in a "reverse running total". I'm quiet unsure of how to do that without un-organizing my list and would love some help figuring it out.

Thanks a bunch!

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Emma Snowden
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Summary Report - Reverse Running Total?