EXCEL HELP

Hello,
I am trying to use excel to work out a statement for me as follows

I want a rolling average percentage based on the current month

I need the reference cell to look at the first month and place it in the ROLLING AVERAGE CELL RESULT
The as the next month gets data to add that month together then divide by 2 ( the number of months ) then when data is entered in the next month take that data and add it up then divide it by the number of months and so on for the next twelve months
I have used this for the first month


=IF(E4<0,0,(E4+F4)/2)