Summing up QTY on Hand

I am developing a report where I have qty on hand. I am putting it into a report that reports sales and need to show qty on hand for all locations. Example. I have stores numbered 1,2,3,4,5 but only stores 1 and 2 have sales and the report shows that they sold 3 each for a total of six size 9 shoes sold. However, the qty on hand field needs to show the qty on hand in all stores. So, if each store has 5 size 9 shoes left, this field should 25 size 9 shoes left.

I am sure I need to write a formula to pull all stores inventory into some sort of variable, then total it and put it on the report, but I can't get my brain around it. Any help would be appreciated.