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.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		int K = sc.nextInt();
		
		TreeSet<String> S = new TreeSet<>();
		TreeSet<String> T = new TreeSet<>();
		
		String[] t = s.split("");
		for(String i : t) {
			T.add(i);
		}
		
		for(String i : T) {
			int x = s.indexOf(i);
			int y = 0;
			while(x != -1) {
				for(int j = x; j < s.length(); j++) {
					for(int k = j; k < s.length(); k++) {
						S.add(s.substring(j, k + 1));
					}
				}
				y++;
				x = s.indexOf(i, y);
			}
			if(S.size() >= K) {
				break;
			}
		}
		
		int a = 0;
		
		for(String i : S) {
			a++;
			if(a == K) {
				System.out.println(i);
			}
		}
	}
}