How to add values in the group's footer

[Kem Arda]

Let me explain:

Consider this dataset returned:

                  Column1         ParentID       SUMFIELD
                        A                         1                10
                        B                         1                10
                        C                         1                10

                        X                          2                15
                        Y                          2                15

I group by ParentID  and wantto display records as follows:

ParentID      
1
2
-------------------
SUMFIELD(10 + 15)        // here is pageFooter of the record

since all SUMFILED fields are same for the records of the same parent id, i just wantto get only one of from each group and add it with other ones from other groups in the page footer. So how can i do that? When i say  SUM(Fields.SUMFIELD) at the page footer it adds 10 + 10 + 10 + 15 + 15.

Thanks in advance...