Context

Here column C contains a running total of the last 5 items from column B.

Formula

The formula is straightforward:

Question

In the above, it's hardcoded for a running total of 5.

What's a good way to have the running total length specified in a separate cell instead of being hardcoded to 5?

I.e. something like this:

so that the user can easily change the 5 to a 7 to get a running total of the last 7.

Link to doc

Link to the spreadsheet shown above.