previous period calculation problem

[#1]

hello,

I have this expression

"sum ({$ <LegalEntity_id_prev_changed={1}>} Salaire_Paye)"

tells me that the wages of a fact.

I would like to calculate the salary of the previous period with the following expression:

sum ({$ <Personn_Matricule = p ({$ <LegalEntity_id_prev_changed = {1}>} Personn_Matricule) Period_Date = {"<= $ (= max (addMonths (Period_Date, -1)))"}>} Salaire_Paye)

it operates over a month but when I select two, she calculates the wage period and made ​​two non period by period ...

you have an idea?

thank you very much