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.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Text;


class Program
{
    static void Main(string[] args)
    {
        // 入力
        var s = Console.ReadLine();
        var k = int.Parse(Console.ReadLine());
        //var n = int.Parse(Console.ReadLine());
        //int[] n = Console.ReadLine().Split().Select(int.Parse).ToArray();

        var mysl = new SortedList(StringComparer.OrdinalIgnoreCase) { };
        for (int i = 0; i < s.Length; i++)
        {
            for (int t = i + 1; t <= s.Length; t++)
            {
                if (s.Substring(i, t - i).Length < 6)
                {
                    if (!mysl.ContainsKey(s.Substring(i, t - i)))
                    {
                        mysl.Add(s.Substring(i, t - i), 1);
                    }
                }
            }
        }

        var ans = mysl.GetKey(k - 1);
        // 出力
        Console.WriteLine(ans);
    }
}