Moving average/average function/adjustable length

I want to use the average function but that the number of cells/dates included to be dependent on the number in another cell (am using it to make a moving average and I want to be able to adjust the number of days in that moving average) - so the last cell in the calculation will always be the same, but the first one not...I dont know if this makes sense but would be happy if someone could help

Hi - is this what you're after
it assumes that the data are in column A begining in A1, and the number of cells to average is in cell B2:

=AVERAGE(A1:OFFSET(A1,B1,0))

Thank you guys, I used the offset function and it worked just fine