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: Accepted)

import java.util.*;

public class Main {
    static int k;
    static Set<String> set;
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        k = sc.nextInt();
        set = new TreeSet<String>();
        int n = str.length();
        
        String[] array = new String[n];
        for (int i = 0; i < n; i++) {
            array[i] = String.valueOf(str.charAt(i));
        }
        
        solve("", array);
        
        int i = 1;
        for (String s : set) {
            if (i == k) {
                System.out.println(s);
                break;
            }
            i++;
        }

    }
    
    public static void solve(String str, String[] array) {
        Set<String> privateSet = new TreeSet<String>();
        
        for (int i = 0; i < array.length - (str.length() - 1); i++) {
            String str2 = "";
            for (int j = i; j < i + str.length(); j++) {
                str2 += array[j];
            }
            
            if (str.equals(str2) && i + str.length() < array.length) {
                privateSet.add(str2 + array[i + str.length()]);
            }
        }
        
        for (String s : privateSet) {
            set.add(s);
            if (set.size() >= k) return;
            solve(s, array);
        }
    }
}