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 JavaScript (node.js v5.12) (judge verdict: Accepted)

"use strict"

const main = arg => {
  arg = arg.trim().split("\n");
  const str = arg[0];
  const len = str.length;
  const k   = parseInt(arg[1]);

  const initials = [...new Set(str.split(''))].sort();
  let index = 0;
  const dic = new Set();

  while(dic.size < k) {
    for(let i=0; i<len; i++) {
      let j = 1;
      if(str.slice(i, i + j) !== initials[index]) continue;

      while(i + j <= len && j <= 5) {
        dic.add(str.slice(i, i + j));
        j += 1;
      }
    }
    
    index++;
  }
  
  const ans = [...dic].sort()[k - 1];
  console.log(ans);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));