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)

function main(input) {
	const args = input.split('\n');
	const s = args[0];
	const K = parseInt(args[1], 10);
	var t = [];
	for(i=0;i<=s.length-1;i++){
		for(j=i+1;j<=i+5;j++){
			var x = s.slice(i,j);
			var d = 0;
			for(k=0;k<t.length;k++){
				if(t[k] === x){
					d = 1;
					break;
				}
			}
			if(d === 0){
				t.push(x);
				t.sort(function(a,b){
					if( a < b ) return -1;
					if( a > b ) return 1;
					return 0;
				});
				if(t.length >= 6){
					t = t.slice(0,5);
				}
			}
		}
	}
	var b = 0;
	var c = '';
	//console.log(t);
	for(i=0;i<t.length;i++){
		if(t[i] != c){
			b += 1;
			c = t[i];
		}
		if(K == b){
			console.log(c);
			break;
		}
	}
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));