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: Runtime Error)

using System;
using System.Linq;
using System.Collections.Generic;
using static System.Console;

class Program
{
    static void Main(string[] args)
    {
        string s = ReadLine();
        int k = int.Parse(ReadLine());

        char first = s.ToCharArray().OrderBy(c => c).First();

        var target = new List<String>();
        Sub(ref target, s, first);
        string str = target.OrderBy(a => a).First();

        var list = new List<String>();
        for (int i = 0; i < str.Length; i++)
        {
            for (int j = 1; j <= str.Length - i; j++)
            {
                list.Add(str.Substring(i, j));
            }
        }

        list = list.Distinct().OrderBy(b => b).ToList();
        WriteLine(list[k - 1]);
    }

    private static void Sub(ref List<String> list, string s, char first)
    {
        int idx = s.IndexOf(first);
        if (s.Length >= 5) {
            list.Add(s.Substring(idx, 5));
        } else {
            list.Add(s);
        }

        if (s.Substring(idx + 1).IndexOf(first) > -1)
        {
            Sub(ref list, s.Substring(idx + 1, s.Length - (idx + 1)), first);
        }
    }

}