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: Wrong Answer)

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
	class Program
	{
		static void Main(string[] a_asArgs)
		{
			string s = Console.ReadLine();
			int K = int.Parse(Console.ReadLine());

			SortedList<string, int> l_List = new SortedList<string, int>();

			for (int l_iStart = 0; l_iStart < s.Length; l_iStart++)
			{
				for (int l_iLen = 1; l_iLen < s.Length - l_iStart + 1; l_iLen++)
				{
					string substring = s.Substring(l_iStart, l_iLen);
					if (l_List.ContainsKey(substring))
					{
						continue;
					}
					if (l_List.Count >= K && substring.CompareTo(l_List.Keys[K - 1]) < 0)
					{
						continue;
					}
					l_List.Add(substring, 0);
				}
			}
			Console.WriteLine(l_List.Keys[K - 1]);
		}
	}
}