User-defined formula - count cells

Hi all,
I set up a formula calculating average annual growth rate (see below).

Period is the number of cells in the range between cells with Y0 (basis year) and Yt (current year) - the cells are in one raw and contain time series data. Currently, I have to count the number of cells by myself and enter it into the formula.

How can I change the formula to make Excel count the number of cells?

Thanks in advance,
Dmitry

Function CAGR(Yt As Variant, Y0 As Variant, Period As Variant)
'Keyword Compound Annual Growth Rate (CAGR)
CAGR = (Yt / Y0) ^ (1 / Period) - 1
End Function