Pivot table (How to add Calculated Field with the difference between the two columns depending on the choice of columns)

[Manojlo]

I made a Pivot table, for example, with 6 columns, period January-Jun. I want to add as last column calculated field, with options (or filter) to calculate difference between two columns. For example give me differences between Jun and January or give me the difference between March and April. Is there any idea for solution? To choose in field H3 which differences I would like to see in column H?