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 <algorithm>
#include <math.h>
#include <string>
#include <functional> 
#include <vector>
#include <numeric>
#include <stack>
#include <queue>
#include <utility>
#include <map>
#include <cstdlib>
typedef long long ll;
#define fi first
#define se second
#define SORT(a) sort(a.begin(),a.end())
#define REP(i,n) for (int i = 0;i < (n) ; i++) 
#define MP(a,b) make_pair(a,b)
 
using namespace std;
 
int main(){

    
    string s ;
    int k;
    cin >> s >> k;
    vector<string>  data(k);
    int a =0;

    for(int i=0;i<s.size();i++){
        for(int j=0;j<k&&j+i<s.size();j++){
            string x = s.substr(i,j+1);
            int b = 0;
          	//cout << x;
            for(int l = 0;l<min(k,a);l++){
                if(data[l] == x) b = 1;
            }
            if(b == 0){
            if(a < k){
                data[a] = x;
                a++;
            }else if(a == k){
                SORT(data);
                if(data[k-1] > x)data[k-1] = x;
                SORT(data);
            }else{
                if(data[k-1] > x)data[k-1] = x;
                SORT(data);
            }}
        }
    }

    cout << data[k-1];
}