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 (1.42.0) (judge verdict: Accepted)

#[allow(unused_imports)]
use proconio::input;
use proconio::marker::*;
use std::collections::HashSet;

fn main() {
  input! {
    s: Chars,
    k: usize,
  }
  let base = [
    'a', 'b', 'c', 'd', 'e',
    'f', 'g', 'h', 'i', 'j',
    'k', 'l', 'm', 'n', 'o',
    'p', 'q', 'r', 's', 't',
    'u', 'v', 'w', 'x', 'y', 'z'
  ];
  
  let n = s.len();
  let mut set: HashSet<String> = HashSet::new();
  for c in base.iter() {
    for (i, cc) in s.iter().enumerate() {
      if c == cc {
        let mut ii = i+1;
        let mut temp = c.to_string();
        set.insert(temp.clone());
        while ii < n && ii < i + 5 {
          temp = temp + &s[ii].to_string();
          set.insert(temp.clone());
          ii += 1;
        }
      }
    }
    if 5 < set.len() { break }
  }
  
  let mut result = set.into_iter().collect::<Vec<String>>();
  result.sort();
  println!("{}", result[k-1]);
}