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: Accepted)

import java.util.*;

public class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int n = sc.nextInt();
        Set<String> ts = new TreeSet<>();
        for(int i = 0;i<s.length();i++){
            ts.add(s.substring(i,i+1));
            if(i + 1 < s.length()){
                ts.add(s.substring(i,i+2));
            }
            if(i + 2 < s.length()){
                ts.add(s.substring(i,i+3));
            }
            if(i + 3 < s.length()){
                ts.add(s.substring(i,i+4));
            }
            if(i + 4 < s.length()){
                ts.add(s.substring(i,i+5));
            }
        }
        Iterator it = ts.iterator();
        for(int i = 0;i<5;i++){
            if(i+1 == n){
                System.out.println(it.next());
                break;
            }else{
                it.next();
            }
        }
    }
}