Show difference between two columns of the same field in a pivot table

Hello

I have a report with a "Month" column which is two months as strings, e.g August September.

In the pivot table, I should the count of items in the Values with the "Month" field in the Columns section.
This gives a count for August and a count for September.

is there anyway I can include a calculated field to show the difference between these two, i.e. the September column minus the August column?

Ah ok.
So I see "Month" the field list and when I click it I get "August" and "September" in the items list.
But, adding the formula =September-August, I get error "PivotTable formulas can only refer to items in the same field as the calculated item."

That seems to be the case, so I'm confused.

Sorry, I didn't click Add before clicking ok.
It works! Thank you for saving me.