Beamer Rmarkdown, changing slide count for backup slides

I am trying to implement this question but in an Rmarkdown beamer presentation:

https://tex.stackexchange.com/questions/70448/dont-count-backup-slides

Can anyone help me out? When I try to put the TeX into the markdown, I get all kinds of errors, often:

    ! Missing \endgroup inserted.
<inserted text> 
                \endgroup 
l.437 \end{frame}

pandoc: Error producing PDF
Error: pandoc document conversion failed with error 43
Execution halted

To give example code, I run this in the latex preamble:

\usepackage{appendixnumberbeamer}

Then in the Rmarkdown:

###Slide 1

Text here

###Slide 2

Text here

\appendix

###First extension slide

Text here

####Second Extension Slide

Text here