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 <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>
#define REP(i, n) for(int i = 0; i < (n); ++i)
typedef long long ll;
const ll MOD = 1000000007;
using namespace std;

int main(){
    string s; cin >> s;
    int k; cin >> k; //k番目は高々k文字
    vector<string> sub;
    REP(i, s.length()){
        for(int j = 1; j <= k && i+j <= s.length(); j++){
            sub.push_back(s.substr(i, j));
        }
    }
    
    sort(sub.begin(), sub.end());
    sub.erase(unique(sub.begin(), sub.end()), sub.end());
    cout << sub[k-1] << endl;
    return 0;
}