Pivot Table Difference From (or maybe something else)

[jg10009]

I'm trying to calculate the difference in TY vs LY numbers in a pivot table. My set up is:

Rows:
Client Name
Sale Date (grouping by month)

Columns:
Years (for the TY vs LY comparison)

Sum Values:
Sum of Sales

For example

	2018	2019
Client 1
May	100	1000

<tbody>
</tbody>

I want to insert a field to show the difference so I'm thinking it's a calculated field using difference from but I can't figure out how to make that work. Thanks for any help.