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: Wrong Answer)

use std::io;
use std::str::FromStr;
use std::collections::BTreeSet;

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

fn main() {
    let s: String = input();
    let k: usize = input();
    let mut sub_str: BTreeSet<String> = BTreeSet::new();
    if k == 1 {
        println!("{}", s);
        return;
    }
    for i in 0..s.len() {
        for j in 1..k+1 {
            if i + j >= s.len() {
                break;
            }
            let sub: String = s.chars().skip(i).take(j).collect();
            // println!("{}", sub);
            if sub_str.len() > k && sub_str.iter().last().unwrap() < &sub {
                break;
            }
            sub_str.insert(sub);
        }
    }
    // println!("{:?}", sub_str);
    let mut i = 0;
    for s in sub_str.iter() {
        if i == k - 1 {
            println!("{}", s);
            return;
        }
        i += 1;
    }
}