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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.util.ArrayList;
import java.util.*;

class Main{
        public static void main(String[] args){

                ArrayList<String> array = new ArrayList<String>();
                ArrayList<String> arraySub;
                Scanner scan = new Scanner(System.in);

                //scan
                String substring = scan.nextLine();
                int outNum = scan.nextInt() - 1;

                for(int i = 0; i < substring.length(); i++){
                        String subText = substring.substring(i);

                        for(int j = 0; j < subText.length(); j++){
                                array.add(subText.substring(0, j+1));
                        }
                }

                //hashset
                arraySub = new ArrayList<String>(new HashSet<>(array));

                //sort
                Collections.sort(arraySub);

                //output
                System.out.println(arraySub.get(outNum));

        }
}