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 (1.42.0) (judge verdict: Runtime Error)

use std::io::Read;
// use std::collections::HashSet;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();

    let answer = solve(&buf);

    println!("{}", answer);
}

fn solve(input: &str) -> String {
    let mut iterator = input.split_whitespace();

    let s: Vec<char> = iterator.next().unwrap().chars().collect();
    let k: usize = iterator.next().unwrap().parse().unwrap();

    // let mut substrings: HashSet<String> = HashSet::new();
    let mut substrings: Vec<String> = Vec::new();
    for i in 0..s.len() {
        let mut substring = String::new();
        for j in i..s.len() {
            substring.push(s[j]);
            // substrings.insert(substring.clone());
            substrings.push(substring.clone());
        }
    }
    // let mut substrings: Vec<String> = substrings.clone().into_iter().collect();
    substrings.sort();
    let mut count = 0;
    for i in 0..substrings.len() {
        if i == 0 || substrings[i - 1] != substrings[i] {
            count += 1;
            if count == k {
                return substrings[i].clone();
            }
        }
    }

    String::new()
}