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

class Program
{
    static void Main()
    {/* 
        var NM = ReadLine().Split().Select(int.Parse).ToArray();
        var N = NM[0];
        var M = NM[1];
        var p = ReadLine().Split().Select(int.Parse).ToArray();
        var x = new int[M];
        var y = new int[M];
        for (int i = 0; i < M; i++)
        {
            var r = ReadLine().Split();
            x[i] = int.Parse(r[0]) - 1;
            y[i] = int.Parse(r[1]) - 1;
        }

        var graph = new List<int>[M];
        for (int i = 0; i < M; i++)
        {
            graph[x[i]].Add(y[i]);
            graph[y[i]].Add(y[i]);
        }
        */

        var s = ReadLine();
        var K = int.Parse(ReadLine());

        var h = new HashSet<string>();
        if (s.Length <= 50)
        {
            for (int i = 0; i < s.Length; i++)
            {
                for (int j = 0; j < i + 1; j++)
                {
                    h.Add(s.Substring(j, s.Length - i));
                }
            }

            var a = new List<string>(h);
            a.Sort();
            WriteLine(a[K - 1]);
        }
        else
        {
            for (int i = 0; i < s.Length; i++)
            {
                for (int j = 0; j < i + 1; j++)
                {
                    h.Add(s.Substring(j, s.Length - i));
                }
            }
            var c = 'a';
            for (int i = 0; i < 26; i++)
            {
                if (s.Contains((char)('a' + i)))
                {
                    c = (char)('a' + i);
                    break;
                }
            }
            if (h.Count(x => x[0] == c) >= K)
            {
                var a = new List<string>(h.Where(x => x[0] == c));
                a.Sort();
                WriteLine(a[K - 1]);
            }
        }
    }
}