Does anyone know if it's possible to add (and subtract) the grand totals from two reports?

I have two reports, each showing totals of the figures for a list of stock items. Both reports show totals of the same items... one is stock in and one is stock out. What I'd like to do is deduct stock out from stock in to give me a running total in a new report or page. Any help on how I might set this up would be appreciated. 

Many thanks.