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: Time Limit Exceeded)

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


list_pair = []
i = 0
while i <= len(s):
    for j in range(i+1,len(s)+1):
        list_pair.append(s[i:j])
    i = i + 1

list_pair = list(set(list_pair))



if len(s)<=50:
    list_pair.sort()
    print(list_pair[k-1])
else:
    abc = "abcdefghijklmnopqrstuvwxyz"
    tmp = []

    i = 0
    j = 0
    while len(tmp) < k + 1:
        for list_j in list_pair:

            if abc[i]==list_j[0]:
                tmp.append(list_j)

        i = i+1

    tmp.sort()

    print(tmp[k-1])