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: Wrong Answer)

import java.io.PrintWriter;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;


public class Main {

  private static final Scanner in = new Scanner(System.in);
  private static final PrintWriter out = new PrintWriter(System.out);
  private static final Set<String> set = new TreeSet<>();

  private static long[] nextArray(int n) {
    long[] arr = new long[n];
    for (int i = 0; i < n; i++) {
      arr[i] = in.nextLong();
    }
    return arr;
  }

  public static void main(String[] args) {
    String s = in.next();
    int k = in.nextInt();
    solve(s, k);
    int i = 0;
    for (String string : set) {
      if (i == k - 1) {
        out.println(string);
      }
      i++;
    }
    out.flush();
    out.close();
  }

  private static void solve(String str, int k) {
    for (int i = 0; i < str.length(); i++) {
      for (int j = i + 1; j < i + k && j <= str.length(); j++) {
        set.add(str.substring(i, j));
      }
    }
  }


}