pivot table calculations

I have not done much in pivot tables. But I have designed a pivot table form
that does all I need except for the last step - creating a column to show a
net amount.
That is, I need to subtract…….
For beginning leases, I have sums for three types – new, renewal or
extension – and a total for all beginning leases.
I have the sum of expiring Leases, that’s all I need. I do not care about
the type of lease.
I need to know a net number for each month – number of all beginning leases
minus the total of all expiring leases. I can’t figure out how to get that
figure in the pivot table in Access – for a number of reasons we do not want
to export the data to Excel. Is that the only option? Can anyone help?
Thank you. All help is greatly appreciated.

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TIA
Bibi