Fill down while skipping cells
 

I have a long list of data listed as duplicate data points that need to be averaged. For example, A1 and A2 are duplicates that need to be averaged, A3 and A4 are duplicates that need to be averaged, etc. How can I create an AVERAGE function and then fill it down so that it will add 2 cells each time (as in =AVERAGE(A1+2),(A2+2) but this does not work)? Thanks!