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 PyPy3 (7.3.0) (judge verdict: Wrong Answer)

import math
#import numpy as np
import queue
from collections import deque,defaultdict
import heapq as hpq
from sys import stdin,setrecursionlimit
#from scipy.sparse.csgraph import dijkstra
#from scipy.sparse import csr_matrix
ipt = stdin.readline
setrecursionlimit(10**7)

def main():
    s = input()
    k = int(input())
    ss = set()
    pri = ["z"]*5
    for i in range(len(s)):
        nw = ""
        for sj in s[i::]:
            nw += sj
            if not nw in ss:
                if nw < pri[0]:
                    pri[1:5] = pri[0:4]
                    pri[0] = nw
                elif nw < pri[1]:
                    pri[2:5] = pri[1:4]
                    pri[1] = nw
                elif nw < pri[2]:
                    pri[3:5] = pri[2:4]
                    pri[2] = nw
                elif nw < pri[3]:
                    pri[4:5] = pri[3:4]
                    pri[3] = nw
                elif nw < pri[4]:
                    pri[4] = nw
            ss.add(nw)
    print(pri[k-1])



    return

if __name__ == '__main__':
    main()