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::collections::BTreeSet;

fn main() {
    let s = read_string();
    let mut sc = Scanner::new(&s);

    let s: String = sc.next();
    let k: usize = sc.next();

    let mut set = BTreeSet::new();

    for i in 0..s.len() {
        for j in 1..6 {
            if i+j<=s.len() {
                set.insert(&s[i..i+j]);
            }
        }
    }

    println!("{}", set.iter().nth(k-1).unwrap());
}

// 以下、たなこふ先生からぱくったやつ (´･_･`)

struct Scanner<'a> {
    iter: std::str::SplitWhitespace<'a>,
}

impl<'a> Scanner<'a> {
    fn new(s: &str) -> Scanner {
        Scanner {
            iter: s.split_whitespace(),
        }
    }

    fn next<T>(&mut self) -> T
        where T: std::str::FromStr,
              <T as std::str::FromStr>::Err: std::fmt::Debug,
    {
        let s = self.iter.next().unwrap();
        s.parse().unwrap()
    }
}

fn read_string() -> String {
    use std::io::Read;

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