How to get previous fiscal period(Fiscal Period-1)

Hi ,

According to my business scenario for reporting-(BI-7) , I want to move FISCPER by 1 . The formula for this is ,

Formula: (Restricited key figure)Sales Order $ (Prior)=Char = Fiscal Period - 1

KF = Sales Order $

I also need a variable for fiscal period for user entry -Single value and mandatory.

I specifically have the task of not including FISCVAR newhere into my query design.

Please suggest how to go about implementing the above scenario ?

Thanks