haskell - Get a header from HTTP response

Question

I can't figure out how to get the Content-Length header from the response from a server:

import Network.HTTP.Conduit

main = do 
      headers <- getHeaders "http://fdsfdsfds.fd"

      --??? this doesn't compile and a wrong way to go
      head $ filter (\x -> hContentLength (fst x)) headers

getHeaders :: String -> IO ResponseHeaders
getHeaders url = do
  req <- parseUrl url
  res <- withManager $ httpLbs req
  return $ responseHeaders res

Your suggestions?