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 <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
using namespace std;
#define REP(i, n) for(int i = 0; i < (int)(n); ++i)

string s;
int kth;
int n;

char b[10];

bool go(int i) {
  for(b[i] = 'a'; b[i] <= 'z'; ++b[i]) {
    b[i+1] = 0;
    if(s.find(b) != string::npos) {
      --kth;
      if(kth == 0) {
        cout << b << endl;
        return true;
      }
      bool res = go(i+1);
      if(res) {
        return true;
      }
    } else {
      continue;
    }
  }
  return false;
}

int main(void) {
  cin >> s >> kth;
  n = s.size();
  go(0);
  return 0;
}