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.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Main {

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            String s = sc.next();
            int k = sc.nextInt();

            Set<String> set = new TreeSet<>();
            for (char ch = 'a'; ch <= 'z'; ch++) {
                for (int i = 0; i < s.length(); i++) {
                    if (s.charAt(i) == ch) {
                        for (int j = 1; j <= 5; j++) {
                            if (i + j <= s.length()) {
                                set.add(s.substring(i, i + j));
                            }
                        }
                    }
                }

                if (set.size() >= k) {
                    break;
                }
            }

            // System.out.println(set);
            List<String> list = new ArrayList<>(set);
            String ans = list.get(k - 1);

            System.out.println(ans);
        }
    }

}