Rewrite IF statement using Set Analyzes

[#1]

Hi All.

I need to rewrite following expression to replace IF condition with set expression, but have no idea, how.

if(
  count({$<Year=,Month=,_Date={">$(=Date(AddMonths(MonthStart(max(_Date))-1,-3)))<$(=Date(AddMonths(MonthStart(max(_Date)),0)))"},Flag={1}>
  }  Id_Month)
  =
  count({$<Year=,Month=,_Date={">$(=Date(AddMonths(MonthStart(max(_Date))-1,-3)))<$(=Date(AddMonths(MonthStart(max(_Date)),0)))"}>
  }  Id_Month)
  ,
  sum({$<Year=,Month=,_Date={">$(=Date(AddMonths(MonthStart(max(_Date))-1,-3)))<$(=Date(AddMonths(MonthStart(max(_Date)),0)))"}>
  *<Year=,Month=,RestQty={'>0'}>
  }  Sales)
  ,Null()
)

The goal of expression:

Get sum of Sales for last 3 monthes, where Current Rests Quantity >0 and only if count of monthes with Flag=1 equals count of monthes with sales.

Any help with my issue will be greatly appreciated. Thanks in advance!