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 (2.4.0) (judge verdict: Accepted)

import sys
input = lambda : sys.stdin.readline().rstrip()

s = input()
n = len(s)
k = int(input())
ans = []
used = set()
"""
from collections import defaultdict
d = defaultdict(list)
for i in range(n):
    d[s[i]].append(i)
""" 
for i in range(n):
    start = list(s[i])
    if s[i] not in used:
        ans.append(s[i])
        used.add(s[i])
    for j in range(i + 1, min(i + k, n)):
        start.append(s[j])
        temp = "".join(start)
        if temp not in used:
            ans.append(temp)
            used.add(temp)
ans.sort()
print(ans[k - 1])