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())
ans_list = []

for i in range(len(s)):
    s_list = [s[j:j+i+1] for j in range(len(s)-i)]
    s_set_list = list(set(s_list))
    for m in range(len(s_set_list)):
        ans_list.append(s_set_list[m])

ans_list.sort()
print(ans_list[k-1])
# print(ans_list)


'''
ans_list = []
s_list = [s[i] for i in range(len(s))]
s_set_sorted = sorted(set(s_list))
for i in range(len(s_set_sorted)):
    ans_list.append(s_set_sorted[i])
if k <= len(ans_list):
    ans_list.sort()
    print(ans_list[k-1])
    exit()

s_queue = []
for i in range(len(s)-1):
    s_queue.append(s[i:i+2])
s_queue_set_sorted = sorted(set(s_queue))
for i in range(len(s_queue_set_sorted)):
    ans_list.append(s_queue_set_sorted[i])
if k <= len(ans_list):
    ans_list.sort()
    print(ans_list[k-1])
    exit()

print(s_set_sorted)
'''