Subtract upper row in a pivot table

[#1]

Hi guys!

I'm new in this community and I hope to find a solution to my issue.

I have a pivot table like this

Month	A	B	X
1	10	0	0
2	5	2	8
3	20	1	4

where only Month is a dimension and A and X are expressions.

I want to evaluate X in this mode X(Month) = A(Month - 1) - B(Month).

Can someone explain me how to do that?

[#2]

Ok,

I made a simple example too. Let me make a rewind and explain the case in more details.

The structure of my table is a little bit more complex as you can see in the attached qvw.

My goal is to evaluate the X column as the sum of A and B of the previous month only if the parameters P1 and P2 are 0.

In the qvw I'm evaluating X using set analysis but it (obviously) works only if a month is selected. I need to evaluate X with the explained rules disregarding selections on month.

In the subsequent table the final result that I must reach

Month	P1	P2	Key	In	Out	A	B	X
1	0	0	A	0	10	10	0	-
1	0	0	B	0	4	20	5	-
1	1	1	A	10	0	0	0	0
1	1	1	B	4	0	0	0	0
2	0	0	A	0	20	5	2	10
2	0	0	B	0	4	30	10	25
2	0	1	A	8	0	0	0	0
2	0	1	B	3	0	10	5	0
2	1	0	A	10	0	0	0	0
2	1	1	A	2	0	0	0	0
3	0	0	B	0	30	20	1	40
3	0	1	B	20	0	0	0	0
3	1	0	B	10	0	0	0	0

Thanks to all and sorry for imprecision.