Multiply and Subtract Custom Script

Hello,

i have a form that i am trying to calculate values from multiple fields to return one result in a 'Total' line.

Fields are, Quantity, Unit Price, Discount, and Total.

I need to have the Total field calculate like this

(Quantity * Unit price) - (Discount * Unit Price) = Total

I have been able to get custom script to simply multiply the sections needed but have not successfully been able to subtract them.