Problem:
Score : 300 points

Problem Statement

You are given a string s.
Among the different substrings of s, print the K-th lexicographically smallest one.

A substring of s is a string obtained by taking out a non-empty contiguous part in s.
For example, if s = ababc, a, bab and ababc are substrings of s, while ac, z and an empty string are not.
Also, we say that substrings are different when they are different as strings.

Let X = x_{1}x_{2}...x_{n} and Y = y_{1}y_{2}...y_{m} be two distinct strings. X is lexicographically larger than Y if and only if Y is a prefix of X or x_{j} > y_{j} where j is the smallest integer such that x_{j} \neq y_{j}.

Constraints

1 ≤ |s| ≤ 5000

s consists of lowercase English letters.

1 ≤ K ≤ 5

s has at least K different substrings.

Partial Score

200 points will be awarded as a partial score for passing the test set satisfying |s| ≤ 50.

Input

Input is given from Standard Input in the following format:

s
K

Output

Print the K-th lexicographically smallest substring of K.

Sample Input 1

aba
4

Sample Output 1

b

s has five substrings: a, b, ab, ba and aba.
Among them, we should print the fourth smallest one, b.
Note that we do not count a twice.

Sample Input 2

atcoderandatcodeer
5

Sample Output 2

andat

Sample Input 3

z
1

Sample Output 3

z

Solution in Python (3.4.3) (judge verdict: Wrong Answer)

import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7

def LI(): return list(map(int,input().split()))
def I(): return int(input())
def LS(): return input().split()
def S(): return input()

def main():
  s=S()
  k=I()

  _s=sorted(s)

  l=[]
  l.append('zzzzzzzzzzzzzzzzzzzz')
  l.append('zzzzzzzzzzzzzzzzzzzzz')

  for i in range(3):
    for j in range(len(s)):
      if len(s)>i and _s[i]==s[j]:
        l.append(s[j])

        if len(s)-j>=2:
          l.append(s[j:j+2])

        if len(s)-j>=3:
          l.append(s[j:j+3])

        if len(s)-j>=4:
          l.append(s[j:j+4])

        if len(s)-j>=5:
          print('aaa')
          l.append(s[j:j+5])

  l=list(set(l))
  l.sort()

  return l[k-1]

print(main())