You would need to create a custom rewrite rule that maps not just the post slug but also the cid value, see this article:
And the relevant Codex documentation:
I'm assuming you don't want to use query strings, since in that case it would be as easy as using this URL:
and getting the value this way:
However, keep in mind that the .../param_name/param_value/ syntax is not the best in terms of semantics and SEO. URLs are supposed to be hierarchical.
Some use /param_name:param_value/ instead, although the most "pure" solution would be to use ?param_name=param_value.