Quantity Sold Item Record

I was trying to show the quantity sold on the item record such that we can then show it on our webstore. 

I thought creating a simple custom field on the item record with a formula would do the job.  We have a custom field with the starting inventory so the plan was just use that less quantity available.  No luck.

here is what I pasted in the formula box.  What am I missing?

{custitem_item_quantity}-{quantityavailable}