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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;

public class Main {

	public static void main(String[] args) throws NumberFormatException, IOException {

		// 解説閲覧後

		// 標準入力
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		String s = br.readLine();
		int k = Integer.parseInt(br.readLine());

		HashSet<String> set = new HashSet<String>();
		set.add(s);

		for (int i = s.length() - 1; 0 <= i; i--) {
			for (int j = (k <= s.length() - i ? k : s.length() - i); 0 < j; j--) {
				set.add(s.substring(i, i + j));
			}
		}

		ArrayList<String> list = new ArrayList<String>(set);
		Collections.sort(list);

		System.out.println(list.get(k - 1));

	}

}