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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.util.*;

public class Main {

    int[] inputArray;
    Set<String> set = new HashSet<>();

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    void run() {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int num = sc.nextInt();
        substring(s, s.length());

        List<String> array = new ArrayList<>(set);

        Collections.sort(array);

        System.out.println(array.get(num-1));
    }

    void substring(String s, int num){
        for(int i=1; i<=num; i++){
            for(int j=0; j<num; j++){
                if(j + i-1 < num){
//                    System.out.println(s.substring(j, j+i));
                    set.add(s.substring(j, j+i));
                } else {
                    break;
                }
            }
        }
    }

    void permutation(String s, String ans){
        if(s.length() <= 1){
            System.out.println(ans + s);
        } else {
            for(int i=0; i<s.length(); i++){
                permutation(s.substring(0, i) + s.substring(i + 1), ans + s.charAt(i));
            }
        }
    }
}