how to combine a single period return formula & a multiple period formula?

in my worksheet column A has period #, B values with over 500 rows. In column c I want to calculate the 5 period ave in a single concise formula and drag it down the column.

A B
1 12
2 18
3 20
4 17
5 17.5
6 18.3
down to
...
500 16

I am hoping to shorten what I am currently using because it will be nested inside another formula, to reduce the number of columns needed to get my desired end result.

=(LN(B10/B9)+LN(B11/B10)+LN(B12/B11)+LN(B13/B12)+LN(B14/B13))/5

Thanks in advance

DILIPandey- I guess I wasn't as clear as I wanted to be, it was late at night when I sent it.

my goal is to combine 2 formulas into one so I can nest them into another formula without needed an additional columns.
formula 1 is to find the return for each period using LN(period 2/period 1) to get the return for period 2, period 1 is just the starting point. formula 2 is an average of these returns. my example above is for 5 periods, the 5 period returns/5.

Additional: I would like to use a reference for the number of periods (N) and have the formula add the correct number of period returns and then divide by N. If this is possible it will make the other columns easier to create.
thanks

Not sure why my replies aren't taking but this is my 3rd and last time writing this.

First thanks Lem

Next is to deal with the variability of a N period averages where the cells not used are blank. I will need to be able to drag this down a column.

Example to calculate the average for 5 period returns N=5 (cell C10=5) in C13 is =IFERROR(LN((INDIRECT("b"&$C$10))/IF($C$10>5,"",B8))/$C$10,"")

If we change the N=10 then in C18 it should read =IFERROR(LN((INDIRECT("b"&$C$10))/IF($C$10>5,"",B18))/$C$10,"") and C13,C14,C15,C16,C17 would all be blank.

Disregard my previous post I resolved it and posted an incorrect formula.

Thanks again,
Lem