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.HashSet;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

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

		TreeSet<Integer> chSet = new TreeSet<Integer>();
		for (int i = 0; i < s.length(); i++) {
			int ch = s.charAt(i) - 'a';
			chSet.add(ch);
		}

		Set<Integer> idxSet = new HashSet<Integer>();
		TreeSet<String> strSet = new TreeSet<String>();
		for (int nextCh : chSet.toArray(new Integer[0])) {
			for (int i = 0; i < s.length(); i++) {
				char ch = s.charAt(i);
				if (ch - 'a' == nextCh) {
					idxSet.add(i);
				}
			}

			for (int i = 1; i <= 5 && idxSet.size() > 0; i++) {
				for (int idx : idxSet.toArray(new Integer[0])) {
					if (idx + i <= s.length()) {
						String str = s.substring(idx, idx + i);
						strSet.add(str);
					} else {
						idxSet.remove(idx);
					}
				}
			}

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

		System.out.println(strSet.toArray(new String[0])[k - 1]);
	}
}