Runnig Sum on report

Hello guys,
I am new to Crystal Report. I would like to show the running sum or cummulative data in the report like..

Item Desc Qty Bal_Qty
AAA Open 10 10
AAA Purchase 5 15
AAA Sales 3 12
AAA Sales Return 2 14
AAA Purchase Return 2 12
AAA Sales 10 2
AAA Purchase 5 7

BBB Open 10 10
BBB Purchase 5 15
BBB Sales 3 12
BBB Sales Return 2 14
BBB Purchase Return 2 12
BBB Sales 10 2
BBB Purchase 5 7

Always ready 2 learn
Kabindra Bakey