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::cmp;

fn main() {
    let s: String = read();
    let k: i32 = read();

    //let mut a = s.chars().collect::<Vec<char>>();
    let mut v: Vec<&str> = (0..s.chars().count()).map(|x| {&s[x..]}).collect();
    v.sort();
    let mut v: Vec<&str> = v[0..cmp::min(k as usize, s.chars().count())].to_vec();

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

    for i in v {
        let mut v1: Vec<&str> = (0..i.chars().count()).map(|x| {&i[..(x+1)]}).collect();
        vv.append(&mut v1);
    }

    vv.sort();
    vv.dedup();

    println!("{}", vv[(k-1) as usize]);
}

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