FMS for Subtraction of Two Fields Value

Hi Experts,

I had made 3 UDF (U_AS_PER_CHALLAN, U_Rec_Qty,U_APPROVE_QTY) by name of As per Challan, Received Quantity & Approved Qty in row level of GRPO

I want FMs should be

As per challan - Received Quantity = Approved Quantity

The final Value should be printed in the third UDF's i.e. Approved Quantity

pls guide me for this fms.

Thanks & Regards,

Pankaj Sharma