how to store running sum of group header fields

HI All,

        I am bit confused how to do this so writing the post here. I have a group header which gets the amount spent by cost centre by account number

the group contains the following fields

[Account Number] [SDESCR] [sum of coscentre1] [sum of costcentre2] and so on.

The end result i get as follows

Account number  Description     Coscentrenumber1  CostCentrenumber2 and so on

100                       cash at bank     6000                        7000

200                           

and so on

now i want

                                      Total    Sumof(Costcentrenumber1)   Sumof(Concentrenumber2)

how can I achieve this ? am confused how to store the value of [sum of costcentre1] into a shared variable and use this shared variable in page footer to display the total ?