Help With This Formula

[#1]

Hi everyone, I have this formula:

sum( {$<Año = {$(=max(Año) - $(=if(max(Mes) = 1, 1, 0)))}, Mes = {$(=if((Mes) = 1, 12, only(Mes)-1) )}, MonthYear = {"<=$(=max MonthYear))"}>} SALES)

I use it to calculate the sales of the previous month of the selected one. My questions is what i have to put in my formula to calculate the salews of the previous months when are more than one selected????, example

There are selected Mar 10, Feb10, Ene 10 and i need to calculate DIC 09, NOV 09 and OCT 09

Any ideas will be appreciate because i'm getting crazy with this issue

Regards

Carlos