multiply qty by amount to get total.

I'm sure this is a simple question but I have a form that produces an amount. The user has a drop box that provides the qty. I would like to multiply amount by qty and get a total.

field names are {qty} and {amount}. I'm simply going to pass this total in an email that is being sent out. I have tried using js but just can't seem to figure it out.

Thanks

Chronoforms V4