How to fetch last six month quantity based on selection inputs in sap bi report

Hi all,

  Please help me.

  We need to calculate one average field based on cal month/ year input.

1. If we run the report one year, we need to fetch last six month quantity and result= (sum of last six month/6)

2. If we run the report less than six month, we need to fetch all the month quantity and result= (sum of all the month/No.of month)

Thanks& Regards,

Alagu.