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 C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics; 

class _Class
{
  void _Do()
  {
    var s = Split.strings()[0];
    var K = Split.ints()[0];

    var hs = new HashSet<string>();

    for(int i=0;i<s.Length;i++ )
    {
      for(int ii=i;ii<s.Length;ii++ )
      {
        hs.Add(s.Substring(i,ii-i+1)); 
      }
    }

    var ss = new SortedSet<string>(hs);
    //
    Console.WriteLine(ss.ElementAt(K-1));
   }

  //
  static void Main(string[] args)
  {
    _(args);
    new _Class()._Do();
  }
  [Conditional("DEBUG")]
  static void _(string[] args)
  {
    if (args.Length != 0 && File.Exists(args[0]))
      Console.SetIn(new StreamReader(args[0]));
    else
      Console.Error.WriteLine("in");
  }
}

static class Split
{


  // 1
  public static string[] strings()
  {
    return Console.ReadLine().Split();
  }


  // [2.1] -2,147,483,648 ~ +2,147,483,647
  public static int[] ints()
  {
    var _s = Console.ReadLine().Split();

    var x = new int[_s.Length];
    for (var i = 0; i < _s.Length; i++)
      x[i] = int.Parse(_s[i]);

    return x;
  }

  // [2.2] 
  // -9,223,372,036,854,775,808 ~ 
  // +9,223,372,036,854,775,807
  public static long[] longs()
  {
    var _s = Console.ReadLine().Split();

    var x = new long[_s.Length];
    for (var i = 0; i < _s.Length; i++)
      x[i] = long.Parse(_s[i]);

    return x;
  }

  public static double[] doubles()
  {
    var _s = Console.ReadLine().Split();

    var x = new double[_s.Length];
    for (var i = 0; i < _s.Length; i++)
      x[i] = double.Parse(_s[i]);

    return x;
  }

  public static BigInteger[] bigs()
  {
    var _s = Console.ReadLine().Split();

    var x = new BigInteger[_s.Length];
    for (var i = 0; i < _s.Length; i++)
      x[i] = BigInteger.Parse(_s[i]);

    return x;
  }
}