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 C++14 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include <bits/stdc++.h>
#define range(i, a, b) for(int i = (a); i < (b); i++)
#define rep(i, a) range(i, 0, a)
using namespace std;

int main() {
    string s;
    int k;
    cin >> s >> k;
    set <char> stc;
    for (auto & ch : s) stc.insert(ch);
    auto itr = stc.begin();

    set <string> sts;
    while ( true ) {
        auto fd = s.begin();
        while (true) {
            fd = find(fd, s.end(), *itr);
            if (fd == s.end()) break;
            string tmp = "";
            for (auto va = fd; va != s.end(); va++) {
                tmp += *va;
                sts.insert(tmp);
            }
            fd++;
        }
        if (sts.size() >= k) {
            auto tmp = sts.begin();
            rep (i, k - 1) tmp++;
            cout << *tmp << endl;
            return 0;
        }
        itr++;
    }
    return 0;
}