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)

fn main() {
    let s = getline();
    let k: usize = getline().trim().parse().unwrap();

    let mut result: Vec<&str> = Vec::new();

    for i in 0..s.len() {
        for size in 1..(k + 1) {
            if i + size < s.len() {
                //println!("{} {} {}", i, size, s[i..(i + size)].to_string());
                result = update_vec(result, s[i..(i + size)].as_ref(), k);
            }
        }
    }
    println!("{}", result[k - 1]);
}

fn update_vec<'a>(mut vec: Vec<&'a str>, string: &'a str, k: usize) -> Vec<&'a str> {
    if vec.len() == 0 {
        vec.push(string);
    } else if vec.contains(&string) {
        // do nothing
    } else {
        for i in 0..vec.len() {
            if string < vec[i] {
                vec.insert(i, string.clone().as_ref());
                break;
            }
        }
        if vec.len() < k {
            vec.push(string);
        } else if vec.len() > k {
            vec.remove(k);
        }
    }
    //println!("{:?}", vec);
    return vec;
}


fn getline() -> String {
    let mut __ret = String::new();
    std::io::stdin().read_line(&mut __ret).ok();
    return __ret;
}