Running total of a group

Hi Everyone,

I am currently using VS 2010 and SAP crystal reports for VS 2010. Please see report designed below.

Enveloppes (Group 1 header)

Enveloppe-A4 (Group 2 header)

Date              Code            RefNo     Qty

2013-09-01     Purchase     P001       100

2013-09-02     Issue           I001          50

2013-09-10     Purchase     P002         25

                                      Remaining  75

The only part I can't do is the summing up of the qty field, since I am supposed to subtract when there is an issue. Am i supposed to create a running total using a formula?

Any advice would be greatly appreciated.

thanks