Dynamic Range and Average Formula

Hi guys,
I'm a newbie in VBA and I can't solve the problem below, can you please help me?
Here a brief description:
in Column A, there's a series of numeric data, from A1 to A31;
in cell C1 I can insert the number that expresses the lenght of the average I'd like to calculate;
in cell C16 the result of the average.
So, the lenght of the average depends on cell C1. The average is a centered average, the maximum range goes from A1 to A31 (if I put 31 in C1. If I put, for example, 5, the range goes from A14 to A18).
Thank you for your help.
Ciao

Marco from Italy