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

namespace AtCoder
{
	public class ABC
	{
		static public void Main(string[] args)
		{
			Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
			Solve();
			Console.Out.Flush();
		}

		const int mod = 1000000007;  // 10^9+7
		static int Mod(long x, int m) { return (int)((x % m + m) % m); }
		static long Mod(long x, long m) { return (x % m + m) % m; }

		static public void Solve()
		{
			//string s = Console.ReadLine();
			//int n = int.Parse(Console.ReadLine());
			//long x = long.Parse(Console.ReadLine());
			//int[] vs = Console.ReadLine().Split().Select(int.Parse).ToArray();
			//long[] vs = Console.ReadLine().Split().Select(long.Parse).ToArray();

			string s = Console.ReadLine();
			int k = int.Parse(Console.ReadLine());

			HashSet<string> map = new HashSet<string>();
			for (int j = 0; j < s.Length; j++) {
				for (int i = 1; i <= s.Length - j && i <= 5; i++) {
					//System.Diagnostics.Debug.WriteLine($"{j},{i} = {s.Substring(j, i)}");
					map.Add(s.Substring(j, i));
				}
			}
			string ans = map.OrderBy(p => p, StringComparer.Ordinal).Skip(k - 1).First();
			Console.WriteLine(ans);

		}
	}
}