I searched this but didn't find the right answer. It's very simple but I don't know formulas well.

I want to keep a running average, meaning as i add new values (in this case my fuel mileage per gallon) i want to know the average of my previous averages. Right now i use =average(a1,a2). but i want a formula that will average a1, a2, a3, a4, a5 etc as i add them so that i don't have to manually rewrite the formula. I know simple enough.

Thanks.