Add a number to a parameter

Question

I'm doing slides with beamer using the mtheme. This theme includes the slide number in the bottom of the page using this code:

\begin{beamercolorbox}[wd=\textwidth,ht=3ex,dp=3ex,leftskip=0.3cm,rightskip=0.3cm]{structure}%
  \hfill\usebeamerfont{page number in head/foot}%
  \insertframenumber%
\end{beamercolorbox}%
}

I wanted to the number to appear as x/X, being X the total number of slides, which can be displayed via \inserttotalframenumber. So i change the third line above to:

\insertframenumber/\inserttotalframenumber%

Now, the problem is that I have a slide at the end that should NOT count in the total number of frames, and so the result is that it's showing x/X, begin X one unit bigger than it should.

How could I do it so it displays \inserttotaframenumber-1. I saw calc allows for simple calculations but using the package and writing just \insertotalframenumber-1 does not work.

Can I do this some how?

Thank you.

Answer 1

You can use a counter:

\newcounter{mynumber}
\setcounter{mynumber}{\inserttotalframenumber}
\addtocounter{mynumber}{-1}
%% Now use:
\insertframenumber/\themynumber