How do I exclude fields from calculated fields?

I have created an order form with the fields: QTY, ITEM and UNIT PRICE and DESCRIPTION with 20 rows. I have tried both PDF menu and simplified field notation to multiply the fields QTY and UNIT PRICE with UNIT PRICE being the only calculated field. The calcualtion works in Row 1, but when I enter any value in Row 2-20'S  QTY or ITEM fields it's calcuated in the UNIT PRICE field in Row 1 increasing it's value. How do I exclude these fields from the calculation? Thanks!