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 static Input;

public class Prog
{
    private const int INF = 1000000007;
    public static void Main()
    {
        string s = Console.ReadLine();
        int K = Cin<int>();
        SortedSet<string> set = new SortedSet<string>();
        for (int i = 1; i <= 5; i++)
        {
            for (int j = 0; j < s.Length - i + 1; j++)
            {
                var ss = s.Substring(j, i);
                if (!set.Contains(ss)) set.Add(ss);
            }
        }
        Console.WriteLine(set.ElementAt(K-1));
    }
}

public class Input
{
    private static Queue<string> queue = new Queue<string>();
    public static T Cin<T>()
    {
        if (queue.Count == 0) foreach (var s in Console.ReadLine().Split(' ')) queue.Enqueue(s);
        if (typeof(T) == typeof(int)) return (T)(object)int.Parse(queue.Dequeue());
        else if (typeof(T) == typeof(long)) return (T)(object)long.Parse(queue.Dequeue());
        else if (typeof(T) == typeof(string)) return (T)(object)(queue.Dequeue());
        else if (typeof(T) == typeof(double)) return (T)(object)double.Parse(queue.Dequeue());
        else return (T)(object)(-1);
    }
    public static T Line<T>()
    {
        if (typeof(T) == typeof(int[]))
        {
            return (T)(object)Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        }
        else if (typeof(T) == typeof(long[]))
        {
            return (T)(object)Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        }
        else if (typeof(T) == typeof(string[]))
        {
            return (T)(object)Console.ReadLine().Split(' ').ToArray();
        }
        else if (typeof(T) == typeof(double[]))
        {
            return (T)(object)Console.ReadLine().Split(' ').Select(double.Parse).ToArray();
        }
        else return (T)(object)(-1);
    }
}