How to calculate with metadata in footer

Question

I have footer that draws on a svn version data field (package: svn-multi). I would like to perform a simple calculation that subtracts a number from the reported version.

The footer code is \lfoot{Ver: \textcolor{red}{1:}\textcolor{red}{\svnkw{LastChangedRevision}}}

Currently \svnkw{LastCHangedRevision} is set at 286 so for this and future reports I need it to present 'LastCHangedRevision-285'

In response to MWE request:

\documentclass[a4paper]{article}

% Versioning
\usepackage{svn-multi}
\svnidlong {$HeadURL:  $}{$LastChangedDate: 2012-02-21 10:20:32 +0000 (Tue, 21 Feb 2012) $}{$LastChangedRevision: 287 $}{$LastChangedBy: $}
\svnid{$Id: BESA-2012.tex 287 2012-02-21 10:20:32Z $}

\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{color, lastpage}
\renewcommand\headrulewidth{0.0pt} \renewcommand\footrulewidth{0.2pt}
\lhead{} \chead{} \rhead{}
\cfoot{\svnkw{LastChangedDate}} \lfoot{Ver:
\textcolor{red}{1:}\textcolor{red}{\svnkw{LastChangedRevision}}}
\rfoot{\thepage/\pageref{LastPage}}

\begin{document}

Answer 1

You can use the \numexpr command:

\lfoot{Ver: \textcolor{red}{1:%
  \number\numexpr\svnkw{LastChangedRevision}-285\relax}}