netting the open quantity

Hi Guys,

I have PO quantity(EKPO-MENGE) and goods receipt quantity(MSEG-ERFMG). Here i want to calculate the open PO quantity which is EKPO-MENGE - MSEG-ERFMG .But here if multiple material docements r getting created for the same for one PO and partial goods receipt quantity is done thru these multiple material documents. then how to net the open quantity.

For example :

PO no PO quantity material documents GR quantity

10 10 100 1

200 2

300 -2

400 3

Now i need the open quantity as 9

7

9

6.

Even i tried with At new Ebeln.but i am not getting it.

Can anybody help me in this.