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;

namespace AtCoder.ABC097
{
    class c
    {
        static void Main(string[] args)
        {
            var s = Console.ReadLine().Trim();
            var line = Console.ReadLine().Trim();
            var K = int.Parse(line);
            var list = new string[K];
            for(var i = 0; i < s.Length; ++i)
            {
                for(var j = 1; j <= s.Length - i && j <= K; ++j)
                {
                    var str = s.Substring(i,j);
                    for(var k = 0; k < K; ++k)
                    {
                        if (list[k] == default(string))
                        {
                            list[k] = str;
                            break;
                        }
                        else
                        {
                            var equalFlg = false;
                            
                            for(var l = 0; l < K; ++l)
                            {
                                if (list[l] == str)
                                {
                                    equalFlg = true;
                                    break;
                                }
                            }
                            if (equalFlg) break;
                            if (list[k].CompareTo(str) > 0)
                            {
                                for (var l = K - 1; l > k; --l)
                                {
                                    list[l] = list[l - 1];
                                }
                                list[k] = str;
                            }
                            else continue;
                        }
                    }

                }
            }
            Console.WriteLine(list[K - 1]);
        }
    }
}