Inverse FizzBuzz

import Control.Monad
import Data.List
import Data.Maybe

iFizzBuzz :: [String] -> Maybe (Int, Int)
iFizzBuzz inp = listToMaybe . map snd . sort $ do
  start <- [1..15]
  let (pos, fbs) = unzip . take (length inp) $ do
        x <- [start..]
        let fizz = x `mod` 3 == 0
            buzz = x `mod` 5 == 0
        guard $ fizz || buzz
        return (x, (if fizz then "fizz" else "") ++ (if buzz then "buzz" else ""))
  guard $ inp == fbs
  return (last pos - head pos, (head pos, last pos))

main = print . iFizzBuzz . words =<< getContents