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 Rust (1.15.1) (judge verdict: Accepted)

use std::io;
use std::cmp;
use std::collections::HashSet;

fn read_line() -> String {
	let mut line = String::new();
	let _ = io::stdin().read_line(&mut line);
	line.trim().to_string()
}

fn rd() -> Vec<String> {
	let mut vs : Vec<String> = vec![];
	for s in read_line().as_str().split(' ') {
//		println!("A {}", s);
		vs.push(s.to_string())
	}
	vs
}

fn to_i(s : &str) -> Result<usize, std::num::ParseIntError> {
	s.parse::<usize>()
}

fn main() {
	loop {
		let s = read_line().as_bytes().to_vec();
		if s.len()==0 {
			break
		}
		let k = to_i(&rd()[0]).unwrap();
		let mut h = HashSet::new();
		for i in 0..s.len() {
			for j in 1..k+1 {
				if i+j <= s.len() {
					h.insert(&s[i..i+j]);
				}
			}
		}
//		println!("{:?}", h);
		let mut a : Vec<&[u8]> = h.iter().cloned().collect();
		a.sort();
		println!("{}", String::from_utf8(a[k-1].to_vec()).unwrap());
	}
}