{-# OPTIONS_GHC -O #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ViewPatterns #-}

import Control.Monad
import Data.List
import qualified Data.ByteString.Char8 as B
import qualified Data.Vector as V
import qualified Data.Set as S

data Dat = Dat { str :: B.ByteString, b :: Int, e ::Int}
instance Show Dat where
    show x = B.unpack $ B.drop (b x) $ B.take (e x + 1) (str x)

instance Ord Dat where
    compare x y = compare (subs x) (subs y)
                    where subs dt = B.drop (b dt) $ B.take (e dt + 1) (str dt)

instance Eq Dat where
    x == y = (subs x) == (subs y)
                    where subs dt = B.drop (b dt) $ B.take (e dt + 1) (str dt)

-- sol :: Int -> B.ByteString -> Char -> String
sol n s c =
    if sz < n
        then sol (n-sz) s (succ c)
        else show $ S.elemAt (n-1) sets
    where
        ls = B.length s
        sz = S.size sets
        sets = V.foldl' (\set st -> S.insert st set) S.empty ss
        ss = V.concatMap ps $ V.unfoldr fi 0
        fi o
            | o >= ls  = Nothing
            | B.index s o == c = Just(o, o+1)
            | otherwise        = fi $ o+1
        ps i = V.map (\to -> Dat s i to)  $ V.enumFromN i $ ls-i

main = do
    s <- B.getLine
    n <- readLn
    putStrLn $ sol n s 'a'
