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 Java7 (OpenJDK 1.7.0) (judge verdict: Accepted)

import java.util.*;
import static java.lang.Math.*;

public class Main {
	
	public static void main(String... args) {
		System.out.println(findSubstring(scanString(), scanInt()));
	}
	
	private static String findSubstring(String s, int K) {
		String last = "";
		TreeSet<String> set = new TreeSet<>();
		for (int i = 0; i < s.length(); ++i) {
			if (!last.isEmpty() && last.charAt(0) < s.charAt(i)) {
				continue;
			}
			int jj = Math.min(s.length(), i + K);
			for (int j = i + 1; j <= jj; ++j) {
				set.add(s.substring(i, j));
				last = trim(K, set);
			}
		}
		return set.last();
	}

	private static String trim(int K, TreeSet<String> set) {
		String last = "";
		while(K < set.size()) {
			last = set.pollLast();
		}
		return last;
	}
	
	private static final Scanner SC = new Scanner(System.in);

	private static int scanInt() {
		return SC.nextInt();
	}

	private static  String scanString() {
		return SC.next();
	}
}