Comparison between any two given months

[#1]

I have a straight table which currently shows month on month comparison for sales. What I want to show is the comparison between any given months not just consecutive months. Note in my case my months 1-12 are marked as period.

So currently my expressions are :

Exprerssion 1 :Sum({<PERIOD={$(=Max(PERIOD)-1)}>}  $(vCurrencyField))

Expression 2:  Sum({<PERIOD={$(=Max(PERIOD))}>}  $(vCurrencyField))

Expression 3 : Difference between the two above. 

Thanks

[Gysbert_Wassenaar]

You can use two variables. Create an input box with two variables vPeriod1 and vPeriod2. Change your expressions to use the variables: Sum({<PERIOD={$(vPeriod1)}>}  $(vCurrencyField))