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()
N = len(S)
K = int(input())

S1 = {}
for i, s in enumerate(S):
    if not s in S1.keys():
        S1[s] = [i]
    else:
        S1[s].append(i)
alp = sorted(list(S1.keys()))

c = 0
for a in alp:
    aset = set()
    inds = S1[a]
    for d in inds:
        for i in range(N-d+1):
            aset.add(S[d:d+i])
    if len(aset)-1 > K-c:
        break
    c += len(aset)-1


alist = sorted(list(aset))
for al in alist:
    if al == '':
        continue
    c += 1
    if c == K:
        ans = al
        break

print(ans)