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: Time Limit Exceeded)

// 

/* 参考
http://vivi.dyndns.org/tech/cpp/set.html
*/

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <deque>
#include <vector>
#include <set>
#include <string>
#include <queue>
#include <map>
#include <cmath>
#include <stack>

#define REP(i, n) for(int i = 0; i < (int)(n); i++)

using namespace std;

int main(){
  string s;
  int k;
  set<string> st;
  
  cin >> s;
  cin >> k;

  int len = s.length();
  for(int i = 0; i < len; i++){
    for(int k = 0; (i+k) <= len; k++){
      string work = s.substr(i, k);
      st.insert(work);
    }
  }
  auto itr = st.begin();
  for(int i = 0; i < k; ++i)
    ++itr;
  cout << *itr << endl;

/*  // debug
  for(auto itr = st.begin(); itr != st.end(); ++itr) {
    std::cout << *itr << "\n";      // イテレータの指す先のデータを表示
  }*/
  return 0;
}