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: Time Limit Exceeded)

import java.io.File;
import java.io.IOException;
import java.util.*;
import java.util.Map.Entry;

public class Main{
	
	public static void main(String[] args) throws IOException{
		//File file = new File("input.txt");
		//Scanner in = new Scanner(file);
		Scanner in = new Scanner(System.in);
		
		String s = in.next();
		int K = in.nextInt();
		TreeSet<String> ts = new TreeSet<>((s1,s2)->s1.compareTo(s2));
		
		for(int i = 1; i <= s.length(); i++){
			for(int j = 0; j + i - 1 <= s.length() - 1; j++){
				String ss = s.substring(j, j + i);
				//System.out.print(ss + " ");
				ts.add(ss);
			}
		}
		
		//for(String ss : ts) System.out.print(ss + " ");
		//System.out.println();
		for(int i = 0; i < K - 1; i++){
			ts.pollFirst();
		}
		System.out.println(ts.pollFirst());
	}
}