writing formula

Hi, I'm trying to create a formula for my share trading spreadsheet. It is to calculate the commission paid per contract traded.

If there is only 1 contract the commission is $4.25, if more than 1 contract it is an extra $0.85 per contract

What I'd like to do is have a call that can calculate the total commissions paid if it is one contract traded or multiple contracts.

I trust this makes sense.

Thanks in advance for your input

Anthony