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++ (GCC 9.2.1) (judge verdict: Time Limit Exceeded)

#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
#include <set>
#include <string>
using namespace std;

string s; int K;
void input()
{
    cin >> s >> K;
}

void solve()
{
    int n = s.size();
    set<string> st;
    for (int i = 0; i < n; ++i){
        for (int j = 1; j <= n-i; ++j){
            // cout << s.substr(i,j) << endl;
            st.insert(s.substr(i,j));
        }
    }

    vector<string> vc;
    for (auto iter = st.begin(); iter != st.end(); ++iter){
        vc.emplace_back(*iter);
    }

    sort(vc.begin(),vc.end());
    cout << vc.at(K-1) << endl;
    // for (auto v: vc){
    //     cout << v << endl;
    // }
}

int main()
{
    cin.tie();
    ios::sync_with_stdio(false);
    input();
    solve();
    return 0;
}