import Control.Applicative
import qualified Data.Set as S

f set
  | S.null set = []
  | t=="z"     = h:f set'
  | otherwise  = h:(f $ S.insert t set')
    where
      ((h:t),set') = S.deleteFindMin set

main = do
  n:s <- lines <$> getContents
  putStrLn $ f $ S.fromList $ map (++"z") s