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: Time Limit Exceeded)

<?php
//Request Question
function getq($get_word=null){
	if($get_word===null){
		return str_replace("\n","",fgets(STDIN));
	}else if($get_word===1){
		return explode(" ",str_replace("\n","",fgets(STDIN)));
	}
}

/*		START		*/
$S = getq();
$K = getq();
$Index = array();
array_push($Index,$S);
//Make Index
for ($i=0; $i < strlen($S); $i++) { 
	for ($j=1; $j < strlen($S); $j++) { 
		if(strlen($S)-$j<0)continue;
		$temp = substr($S,$i,$j);
		if(!array_search($temp,$Index))array_push($Index,$temp);
	}
}
$Index=array_merge($Index);
sort($Index);
echo $Index[$K-1]."\n";
?>