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: Time Limit Exceeded)

#![allow(non_snake_case)]

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    s.trim().parse().ok().unwrap()
}

fn main() {
    let s: String = read();
    let K: usize = read();
    let mut hash = std::collections::HashSet::new();
    let chars = "abcdefghijklmnopqrstuvwxyz";
    for c in chars.chars() {
        let mut i = 0;
        loop {
            i = match s[i..].find(c) {
                None => s.len(),
                Some(j) => j+i
            };
            if i == s.len() {
                break;
            }
            for j in i+1..s.len()+1 {
                hash.insert(s[i..j].to_string());
            }
            i += 1;
        }
        if hash.len() >= K {
            break;
        }
    }
    let mut dict: Vec<String> = hash.iter().map(|x| x.to_string()).collect();
    dict.sort();
    println!("{}", dict[K-1]);
}