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: Time Limit Exceeded)

function Main(input){
	input=input.split(/\n|\s/);
	var al=input[0].split("");
	al.sort();
	var arr=[];
	var z=0;
	
	al=al.filter(function(x,i,self){return self.indexOf(x)===i;});
	
	for(var i=0;i<al.length;i=(i+1)|0){
		for(var j=0;j<input[0].length;j=(j+1)|0){
			if(al[i]==input[0][j]){
				for(var k=1;(k<=input[0].length-j);k=(k+1)|0){
					arr[arr.length]=input[0].slice(j,k+j);
					if(k==6){break;}
				}
			}
		}
		arr=arr.filter(function(x,i,self){return self.indexOf(x)===i;});
		if(arr.length>5){break;}
	}
	arr.sort();
	console.log(arr[(input[1]|0)-1]);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));