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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;
import java.util.TreeSet;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CKthSubstring solver = new CKthSubstring();
        solver.solve(1, in, out);
        out.close();
    }

    static class CKthSubstring {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
        /*
        明らかにk番目の文字列の長さはk以下であることが見えているのですくなさそう
        そのままやるだけでは…？
        mleに注意が必要そうってくらいか
         */
            String s = in.next();
            int k = in.nextInt();
            TreeSet<String> ts = new TreeSet<>();
            for (int i = 0; i < s.length(); i++) {
                for (int j = i + 1; j <= s.length(); j++) {
                    if (j - i <= k) {
                        ts.add(s.substring(i, j));
                    }
                }
            }

            for (String t : ts) {
                k--;
                if (k == 0) {
                    out.println(t);
                    break;
                }
            }


        }

    }
}