How to attach different number sequence to Returned Purchase Order

Hi all,

When we create purchase order then I want different PurchID number sequence for Purchase Order and Returned Order Purchase Type.

i.e if we select Purchase type - Purchase Order then I want Purchase order number is- PO-0001

and if we select Purchase type - Returned Order then I want Purchase order number is - RO-0001 i.e different number sequence

If there is any customization for that then please help me how to that and in what classes?

Thanks

Bhushan