Subtracting subgroups

[gaby78]

So far I have always used summary reports where sub groups are added together. In my current situation I need to get partial results ie subtract a subgroup from the preceding subgroup like this:

5

3

6

2

16 ---> subtotal sg1

2

2

3

7 ---> subtotal sg2

9 ---> Net1 (sg1 – sg2)

....

....

5 ---> subtotal sg3

4 ---> Net2 (Net1 – sg3)

..... etc...

I can’t figure out how to do this in a columnar report with subsummaries.