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<bits/stdc++.h>
using namespace std;
typedef long long llint;
typedef long double ld;

#define inf 1e18
#define mod 1000000007
priority_queue<llint,vector<llint>,greater<llint> > que;
priority_queue<llint> Que;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }



void solve(){
  string s;
  cin >> s;
  int k;cin >> k;
  set<string> d;
  for(int i=0;i<s.size()-1;i++){
    for(int j=1;j<=k;j++){
      string t=s.substr(i,j);
      d.insert(t);
    }
  }
  if(d.empty())d.insert(&s[0]);
  auto itr=d.begin();
  for(int i=0;i<d.size();i++){
    if(i==k-1){
      cout << *itr << endl;
      break;
    }
    itr++;
  }
}

int main(){
  solve();
  return 0;
}