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: Wrong Answer)

var fs = require("fs");
 
fs.readFile("/dev/stdin", "utf-8", function(err, data) {
  if(!err) {
    var str = data.split("\n")[0];
    var k = data.split("\n")[1] - 0;
    console.log(Array(str.length).fill().map(function (_, i) {
      return Array(str.length - i).fill().map(function (_, j) {
        return j > 0 ? str.substr(i, j + 1) : str;
      });
    }).reduce(function (_, a) {
      return _.concat(a);
    }).filter(function(s) {
      return s.length <= k;
    }).filter(function (s, i, arr) {
      return arr.filter(function (_s) {
        return _s === s;
      }).length == 1;
    }).sort().reverse()[k-1]);
  } else {
    throw err;
  }
});