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: Wrong Answer)

S = input()
K = int(input())
if len(S) >= K:
    ans = S[:K]
    for i in range(len(S) - K + 1):
        tmp = S[i:i+K]
        if tmp < ans:
            ans = tmp
    print(ans)
elif len(S) == 2:
    if S[0] > S[1]:
        print(S)
    else: print(S[1])
elif len(S) == 3:
    sl = list(set([S[0],S[1],S[2], S[:2],S[1:], S]))
    sl.sort()
    print(sl[K-1])
else:
    sl = list(set([S[0],S[1],S[2],S[3], S[:2],S[1:3],S[2:], S[:3],S[1:], S]))
    sl.sort()
    print(sl[K-1])