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 (3.8.2) (judge verdict: Wrong Answer)

from operator import itemgetter
s = [[string, i] for i, string in enumerate(input())]
k = int(input())

cand = sorted(s, key = itemgetter(0))[:k] if len(s) >= k else sorted(s, key = itemgetter(0))
ans_cand = []
def check(list, k):
    string, ind = list
    temp_s = ''
    for i in range(k): 
        if ind + i > len(s) - 1:break
        temp_s += s[ind + i][0]
        ans_cand.append(temp_s)

for t in cand:
    check(t, k)
    
ans_cand.sort()
pre = ans_cand.pop(0)
k -= 1
while k != 0:
    temp = ans_cand.pop(0)
    if pre == temp:continue
    pre = temp
    k -= 1
print(pre)