IN formula field HOw to check sub group changing in a group

Hi

IN formula field How to check sub group changing in a group

In my report each group divided into subgroup and in each subgroup i am calculating discount on some value

I have a formula field in subgroup its calculating discount of some value

here my problem is i want to show sum of discount values in group footer of each sub group

but in my formula how i check sub group changing

This is the formula in group footer 2

1)

@Discount :

(({tbl_1e.ID_P_Discount} / 100) * {#Pub_TotalAmount})

2)

and i created a formula field in same footer 1 and suprressed

@Footer2_discount

numberVar B:=B+{@Pub_Discount};

3)

In Footer1 in group

WhilePrintingRecords;

{@Footer2_discount}-{@Discount};

here i need sum of every subgroup discount separately for each group