#! /usr/bin/env stack
-- stack --resolver lts-12.20 --install-ghc script
{-# LANGUAGE ViewPatterns #-}
module Main where

import Data.Function (fix)
import qualified Data.List as L
import qualified Data.Set  as S
import qualified Data.Map  as M

main :: IO ()
main = interact $ (. (M.unionsWith (<>) . map parseLine . lines)) $ fix $ \repeat depSets ->
   if M.null depSets then "" else let
      (noDep, _) = M.findMin $ M.filter S.null depSets
      depSets' = M.map (S.delete noDep) (M.delete noDep depSets)
   in noDep : repeat depSets'

parseLine :: String -> M.Map Char (S.Set Char)
parseLine (words -> ws) =
       M.unionWith (<>) (M.singleton req S.empty)
                        (M.singleton step (S.singleton req))
   where
      req  = (head $ ws !! 1)
      step = (head $ ws !! 7)