Using a Count'ed field in a Calculated Formula for a Pivot Table
 

I have created a pivot table with the following:
- A row field called Type
- Data Items called Resource Affected
- No column fields dropped in...but now shows "Count of Resource Affected"

My data table has 2 other fields, Original Date and New Date. I have added
a calculated field which is New Date - Original Date, and I have called it
Person Days Pushed.

I would like to create a calculated field that is:
(Original Date - New Date) / Count of Resource Affected

The problem is that in the calculated fields window, I can only select
Resource Affected... How do I do the formula for the calculated field to be
the count of the resources? Thanks.