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: Accepted)

#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1LL << 30;
int MOD = 1e9+7;
main(){
    string S;
    int N;
    cin >> S >> N;
    set<string> st;
    /*
    bool flag[26] = {};
    for(char c:S){
        flag[c-'a'] = 1;
    }
    queue<int> que;
    int cnt = 0;
    for(int i = 0;i < 26;i++){
        if(!flag[i])continue;
        char next = i + 'a';
        for(int j = 0;j < S.size();j++){
            if(S[j] == next)que.push(j);
        }

    }
    */
    
    for(int i = 0;i < S.size();i++){
        for(int j = 1;j <= 5 && i+j <= S.size();j++){
            st.insert(S.substr(i,j));
        }
    }
    for(string s:st){
        N--;
        if(N == 0){
            cout << s << endl;
            return 0;
        }
    }
}