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::prelude::*;

fn input<T: std::str::FromStr>() -> T {
    let stdin = std::io::stdin();
    let it = stdin.lock().bytes().map(|c| c.unwrap() as char);
    let (fs, ft) = (|c: &char| c.is_whitespace(), |c: &char| !c.is_whitespace());
    let token = it.skip_while(fs).take_while(ft).collect::<String>();
    token.parse().ok().unwrap()
}
#[allow(dead_code)]
fn with_stdout_lock<F: FnOnce(std::io::BufWriter<std::io::StdoutLock>)>(f: F) {
    let out = std::io::stdout();
    f(std::io::BufWriter::new(out.lock()));
}
#[allow(unused_macros)]
macro_rules! debug {
    ($($var:expr),*) => { #[cfg(debug_assertions)] {
        $({ match &$var { v => eprint!("{}={:?} ", stringify!($var), v) } })*
        eprintln!();
    }};
}

fn main() {
    let s: Vec<char> = input::<String>().chars().collect();
    let k: usize = input();

    let mut t = vec![0; 26];
    for &c in &s {
        let i = (c as u8 - b'a') as usize;
        t[i] += 1;
    }

    let mut u = vec![];
    for (i, _) in t.iter().enumerate().filter(|&(_, c)| *c > 0) {
        let c = char::from(b'a' + i as u8);
        for j in 0..s.len() {
            if s[j] != c {
                continue;
            }
            let mut p = 0;
            while j + p < s.len() && p < k {
                u.push(s[j..j + p + 1].iter().cloned().collect::<String>());
                p += 1;
            }
        }
        u.sort();
        u.dedup();
        if u.len() >= k {
            break;
        }
    }

    u.sort();
    debug!(u);
    println!("{}", u[k - 1]);
}