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

namespace ConsoleApp4
{
    class Program
    {
        public static void Main(string[] args)
        {
            List<string> kouho = new List<string>();
            string s = Console.ReadLine();
            int k = int.Parse(Console.ReadLine());
            for (int i = 'a'; i < 'z' + 1; i++)
            {
                int focus = s.IndexOf((char)i);
                while(focus != -1)
                {
                    for (int c = 0; c < 5; c++)
                    {
                        try
                        {
                            if (!kouho.Contains(s.Substring(focus, c + 1)))
                            {
                                kouho.Add(s.Substring(focus, c + 1));
                            }
                        }
                        catch
                        {
                            break;
                        }
                    }
                    if (focus == 0)
                    {
                        s = 'X' + s.Substring(focus + 1);
                    }
                    else if (focus + 1 <= s.Length)
                    {

                        string look1 = s.Substring(0, focus - 1);
                        string look2 = s.Substring(focus + 1);
                        s = s.Substring(0, focus) + 'X' + s.Substring(focus + 1);
                    }
                    else
                    {
                       
                        s = s.Substring(0, focus) + 'X';
                    }
                    focus = s.IndexOf((char)i);

                    if (focus == -1)
                    {
                        s = s.Replace('X', (char)i);
                        break;
                    }
                    else
                    {

                    }

                }
                if (kouho.Count >= 5) break;
            }
            ;
            kouho.Sort();
            ;
            Console.WriteLine(kouho[k - 1]);
        }

    }
}