import Data.List
import Debug.Trace

main = do
  s <- getLine
  k <- read <$> getLine :: IO Int
  putStrLn $ (sort $ nub $ subseq s [])!!k

subseq :: String -> [String] -> [String]
subseq [] acc = acc
subseq s acc=
--  traceShow acc $
  subseq (tail s) $ inits s ++ acc
