How to number slides in the beamer theme Hannover ?

If you only want to have the slide numbers in the footer, I use this (I adapted Matthews code):

\documentclass{beamer}
\title{Test of Hannover with slide numbers}
\usetheme{Hannover}
\setbeamertemplate{footline}[frame number]

\begin{document}
\begin{frame}
\maketitle
\end{frame}

\begin{frame}
\begin{itemize}
\item Abel
\item Baker
\item Charlie
\end{itemize}
\end{frame}

\end{document}

For more details on how to modify beamer templates you can also have a look at the Beamer User Guide

I find a way finally. It is not a light solution, but it looks better.

My code is below. Maybe someone else also need it.

\documentclass{beamer}
\title[Hack Hannover]{Hannover with slide numbers}
\author[Beamer]{Beamer}
\usetheme{Hannover}
\def\swidth{1.6cm}
\setbeamersize{sidebar width left=\swidth}
\setbeamertemplate{sidebar left}
{
  {\usebeamerfont{title in sidebar}%
    \vskip1.5em%
    \usebeamercolor[fg]{title in sidebar}%
    \insertshorttitle[width=\swidth,center,respectlinebreaks]\par%
    \vskip1.25em%
  }%
  {%
    \usebeamercolor[fg]{author in sidebar}%
    \usebeamerfont{author in sidebar}%
    \insertshortauthor[width=\swidth,center,respectlinebreaks]\par%
    \vskip1.25em%
  }%
  \hbox to2cm{\hss\insertlogo\hss}
  \vskip1.25em%
  \insertverticalnavigation{\swidth}%
  \vfill
  \hbox to2cm{\hskip0.6cm\usebeamerfont{subsection in
      sidebar}\strut\usebeamercolor[fg]{subsection in
      sidebar}\inserttotalframenumber-\insertframenumber\hfill}%
  \vskip3pt%
}%

\begin{document}
\begin{frame}
\maketitle
\end{frame}

\section{Introduction}
\begin{frame}
\begin{itemize}
\item Abel
\item Baker
\item Charlie
\end{itemize}
\end{frame}

\section{Conclusions}
\begin{frame}
\begin{itemize}
\item Abel
\item Baker
\item Charlie
\end{itemize}
\end{frame}

\end{document}

You can install an outer theme on top of a complete theme, as in:

\documentclass{beamer}
\title{Test of Hannover with slide numbers}
\usetheme{Hannover}
\useoutertheme{infolines}
\setbeamertemplate{headline}{} % removes the headline that infolines inserts

\begin{document}
\begin{frame}
\maketitle
\end{frame}

\begin{frame}
\begin{itemize}
\item Abel
\item Baker
\item Charlie
\end{itemize}
\end{frame}

\begin{frame}
\begin{itemize}
\item Abel
\item Baker
\item Charlie
\end{itemize}
\end{frame}

\end{document}