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# (.NET Core 3.1.201) (judge verdict: Accepted)

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

class MainClass
{
    static void Main()
    {
        string s = Console.ReadLine();
        var K = int.Parse(Console.ReadLine());

        int loop = 0;
        List<string> vs = new List<string>();
        char fw = 'a';
        while (vs.Count < K)
        {
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == fw)
                {
                    for (int j = 0; j < Math.Min(6,s.Length-i); j++)
                    {
                        string w = s.Substring(i, j+1);
                        if (!vs.Contains(w)) vs.Add(w);
                    }
                }
            }
            fw++;
        }
        vs.Sort();
        Console.WriteLine(vs[K-1]);
        //Console.WriteLine(string.Join(" ",vs));
    }
}