# Is F in P=F/A an average or a sum of all microscopic forces?

Hello,
I've read that the force F we use in the pressure formula P=F/A is an average of all microscopic forces exerted by the gas on the wall of the container over a period of time. Is it true? I thought it was a SUM rather than an average?
I assume that in that definition the microscopic force is a force exerted by a SINGLE particle on the wall.