Unique Values In Pivot Tables

I have a list of purchase order line items. Each Purchase order could have multiple line items, so the list could have many occurances of the same purchase order number. I have designed a pivot table which summaries line items by ordering area (one of the fields in the list).

This works fine but what I would also like to check is the number of purchase orders by area. How can I acheive this? I would prefer not to use any helper columns but will adapt my application if this is needed.