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: Accepted)

using System;
using System.Linq;//リストの使用
using System.Collections.Generic;
class Program
{
	static void Main()
	{
	string s = Console.ReadLine();
  int k = int.Parse(Console.ReadLine());

  List<string> checkList = new List<string>();

    for(int i = 0; i < k; i++)
    {
      for(int j = 0; j < s.Length - i; j++)
      {
        //Console.WriteLine(s.Substring(j, i+1));
        checkList.Add(s.Substring(j, i+1));//j文字目からi文字
      }
    }

    checkList.Sort((a, b) => a.CompareTo(b));//ソート

    string memo = "aaaaaa";
    long check = 0;
    
      for(int i = 0; i < checkList.Count(); i++)
      {
        if(checkList[i] != memo) 
        {
          check ++;
          if(check == k) Console.WriteLine(checkList[i]);
          memo = checkList[i];
        }
      }
    
      
	}
}