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 collections import Counter
def main():
    s=input()
    k=int(input())
    p=sorted(Counter(s).items(), key=lambda x: x[1], reverse=True)
    count = 1
    for i in range(len(p)):
        for t in g(p[i][0], s):
            if count == k:
                print(t)
                return
            count += 1
        
def g(s, ss):
    sumi = set()
    lens = len(ss)
    for l in range(lens):
        for i in range(lens):
            if i+l >= lens:
                break
            res = ss[i:i+l+1]
            if (ss[i] == s) and (res not in sumi):
                sumi.add(res)
    return sorted(sumi)

if __name__ == "__main__":
    main()