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: Runtime Error)

s = input()
k = int(input())

alphabet = [chr(i) for i in range(97, 97+26)]
dictionary = list()
for char in alphabet:
    tmp_s = s
    num = tmp_s.find(char)
    while num != -1:
        for i in range(len(tmp_s)-num):
            dictionary.append(tmp_s[num:num+i+1])

        else:
            tmp_s = tmp_s[num+1:]

        num = tmp_s.find(char)

    if len(dictionary) > 5:
        break

seen = set()
seen_add = seen.add

dictionary = [ x for x in dictionary if x not in seen and not seen_add(x)]
dictionary.sort()
# print(dictionary)
print(dictionary[k-1])