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.Scanner;
import java.util.TreeSet;
import java.util.stream.IntStream;

public class Main {
    public static String process(TestCase testCase) {
        final String s = testCase.s;
        final int K = testCase.K;

        TreeSet<String> set = new TreeSet<>();
        final int length = s.length();
        IntStream.range(0, K + 1)
                .forEach(i ->
                        IntStream.range(0, length - i)
                                .forEach(j ->
                                        set.add(s.substring(j, j + i + 1))
                                )
                );

        return new ArrayList<>(set).get(K - 1);
    }

    // Util func

    public static void main(String[] args) {
        TestCase testCase = readFromInput();

        final String result = process(testCase);

        output(result);
    }

    private static TestCase readFromInput() {
        Scanner sc = new Scanner(System.in);

        String s = sc.next();
        int K = sc.nextInt();

        return new TestCase(s, K);
    }

    private static void output(String result) {
        System.out.println(result);
    }

    public static class TestCase {
        final String s;
        final int K;

        public TestCase(String s, int K) {
            this.s = s;
            this.K = K;
        }
    }
}