Subtract one variable from another.

[cddw.ltd]

Right, I've got two variables,

$totalposts
and
$rssposts

Which shows the posts from the RSS user.

How would I go about creating a third variable which shows $totalposts - $rssposts ?

I've tried creating a new plugin with -

$test = $totalposts - $rssposts;

but wherever I use $test I just get -233 (233 been the posts created by the RSS user.) So it looks like its not pulling $totalposts properly, and having it show as 0.