Calculate After hours

Hi There!

I am having a bit of trouble while creating a formula,
It goes like this:

FIELDS:
A - (START TIME)
Value: Let's say (7 am)

B - (FINISH TIME)
Value: Let's say (10 pm)

C - After hours (consider hours past 6 PM after hours)
Result is (4)

Statement:
If working hours are past 6PM, then calculate after hours, if working hours are not past 6 pm do not calculate it.

What would the formula look like??
Thank you very much!