How to calculate a rolling average over X periods

Hello,

I am new to this forum and had an excel question I am hoping someone can help with. I have one column of data, and in the adjacent column I want to calculate a rolling X-period average from starting X periods back from the cell value. I would then like to define X in a single cell, so that I in one cell I could change X to 3 to get a 3 period average, or change X to 5 to get a 5 period average.

I attached an example spreadsheet of what I am trying to do, if anyone can help it would be much appreciated!!

That's perfect, thanks so much!

-Justin