Ignore selection on the expression but not dimention

[#1]

Hi all,

I have some data in the attached file showing this table:

(The data in the script include Player field, you can look at the attached file)

Month	Team	=Sum(Goals)
Jan	T1	8
Jan	T2	10
Jan	T3	5
Feb	T1	6
Feb	T2	9
Mar	T1	2
Mar	T2	3
Mar	T3	1
Apr	T2	1

I want to change the expression that will present the average of two months from the current month and backward.

I need the table show it per every month (and team) on the table.

The straight table should be like this:

Month          Team          Goals

-------------------------------------------

Jan               T1                4                -> The Average of Jan and Dec is (8+0)/2 = 4

Jan               T2                5                -> The Average of Jan and Dec is (10+0)/2 = 5

Jan               T3                2.5             -> The Average of Jan and Dec is (5+0)/2 = 2.5

Feb               T1                7                -> The Average of Feb and Jan is (6+8)/2 = 7

Feb               T2                9.5             -> The Average of Feb and Jan is (9+10)/2 = 9.5

Feb               T3                2.5             -> The Average of Feb and Jan is (0+5)/2 = 2.5

Mar               T1                4                -> The Average of Mar and Feb is (2+6)/2 = 4

Mar               T2                6                -> The Average of Mar and Feb is (3+9)/2 = 6

Mar               T3                0.5             -> The Average of Mar and Feb is (1+0)/2 = 0.5

Apr               T1                1                -> The Average of Apr and Mar is (0+2)/2 = 1

Apr               T2                2                -> The Average of Apr and Mar is (1+3)/2 = 2

Apr               T3                0.5                -> The Average of Apr and Mar is (0+1)/2 = 0.5

Please help me with this, how do I ignoring the months selection per row

and calculate the average for the specific month each row.

Thanks!