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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <string>
#include <algorithm>
#include <set>

int main() {
    std::string s, ret;
    int k;
    std::cin >> s >> k;

    char smallest[5] = { 'z', 'z', 'z', 'z', 'z' };
    for (auto c : s) {
        for (int i = 0; i < 5; i++) {
            if (smallest[i] >= c) {
                smallest[i] = c;
                break;
            }
        }
    }
    
    std::set<std::string> sset;
    for (int i = 0; i < 5; i++) {
        for (int j = 0; j < s.length(); j++) {
            if (s[j] == smallest[i]) {
                int limit = std::min(5, (int)s.length() - j);
                for (int l = 1; l <= limit; l++){
                    sset.insert(std::string(s, j, l));
                }
            }
        }
        if (sset.size() >= k) break;
    }

    for (auto& str : sset) {
        if (--k == 0) std::cout << str << std::endl;
    }
    
    return 0;
}