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>
using namespace std;

#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep1(i, n) for (ll i = 1; i < (ll)(n); i++)
#define INF 1000000000000
#define MOD 1000000007
using ll = long long;

int main(){
  string s;
  int k;
  cin >> s >> k;
  int len = s.size();

  
  set<string> jisho;
  for(int l = 1; l <= min(k,len); l++){//長さ
    for(int i = 0; i < len-l+1; i++){
      string tmp = "";
      for(int j = 0; j < l; j++){
        tmp = tmp + s.at(i+j);
      }
      jisho.insert(tmp);
    }
  }
  int cnt = 0;
  for(auto str: jisho){
    cnt++;
    if(cnt==k){
      cout<<str<<endl;
      return 0;
    }
  }
  
  return 0;
}