Buy X Get Y Free scheme Logic

Hi,

I have some items with MRP. I want to apply some offer to this items like

Buy 1 Get 1 Free/Buy 2 get 1 Free / Buy 1 Get 2 Free, etc.

Suppose 1 Customer buying 2 items such as,(these items are added in Gridview.)

Itemcode   MRP

001            200

002             120

So if i am applying the Scheme (Buy 1 Get 1 Free), It will apply 100% discount to the Cheaper item i.e. item 002.

Can any one give some logic to appy this.

thanks and regards,

Sanjaya kumar mohapatra