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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php

while ($stdin = fgets(STDIN)) {
    $line[] = trim($stdin);
}
foreach ($line as $key => $value) {
    $data[] = explode(' ', $value);
}

//foreach ($data as &$line) {
//    $line = array_map(function($val) { return (int)$val; }, $line);
//}

$s = $data[0][0];
$K = $data[1][0];

$alphabetKey = 0;
$length = strlen($s);

$founds = [];

for ($smallK = 1; $smallK <= $K; $smallK++) {
    for($start = 0; $start < $length + 1 - $smallK; $start++) {
        $founds[] = substr($s, $start, $smallK);
    }
}

$founds = array_values(array_unique($founds));
sort($founds);

$ans = $founds[$K - 1];
printf('%s', $ans);

?>