Aumming fields based on other fields

I have an inventory database with a quantity on hand field. This field holds information pertaining to each store's location and the warehouse. The inventory application will show quantity on hand throught the organization, but it is a calculated field so I can't just drop it in my report. So, I am trying to figure out how to create a formula to add up the quantity on hand at the store locations selected for the report. So, if I select Stores 1, 2, 3, and the stores have 5, 10, and 15 widgets respectively, I would like to show quantity on hand of 30. To make things even more complicated, I would like to add in my warehouse to quantity on hand or put in a seperate field on the report.