What is the best way to convert a ByteString to an Int?

I always run into the following error when trying to read a ByteString:
Prelude.read: no parse

Here's a sample of code that will cause this error to occur upon rendering in a browser:

factSplice :: SnapletSplice App App
factSplice = do
    mbstr <- getParam "input" -- returns user input as bytestring
    let str = maybe (error "splice") show mbstr
    let n = read str :: Int
    return [X.TextNode $ T.pack $ show $ product [1..n]]

Or perhaps more simply:

simple bs = read (show bs) :: Int

For some reason, after show bs the resulting string includes quotes. So in order to get around the error I have to remove the quotes then read it. I use the following function copied from the internet to do so:

sq :: String -> String
sq s@[c]                     = s
sq ('"':s)  | last s == '"'  = init s
            | otherwise      = s
sq ('\'':s) | last s == '\'' = init s
            | otherwise      = s
sq s                         = s

Then simple bs = read (sq.show bs) :: Int works as expected.

1. Why is this the case?
2. What is the best way to convert a ByteString to an Int?