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 <vector>
#include <math.h>
#include <set>
#include <map>
#include <stack>
#include <queue>
using namespace std;
using ll = long long;
int main(){
    string s;
    int K;
    set<string> st;
    cin >> s;
    cin >> K;
    string str;
    for(int i=0; i < s.size(); i++){
        for(int j=1; j <= s.size(); j++){
            str = s.substr(i, j);
            st.insert(str);
            if(st.size() > K) break;
        }
    }

    /*for(auto itr = st.begin(); itr != st.end(); ++itr) {
        std::cout << *itr << endl;      // イテレータの指す先のデータを表示
    }*/

    auto itr = st.begin();

    for(int i=1; i<K; i++){
        itr++;
    }

    cout << *itr << endl;

    //cout << s.size() << endl;
    /*std::string str("012345");
    std::string sub = str.substr(2, 3);     //  [2] の位置から3文字切り出す
    std::cout << sub << "\n";   234*/ 
    return 0;
}