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::io::{self, BufRead};
use std::cmp::min;
use std::collections::BTreeSet;

fn main() {
  let stdin = io::stdin();
  let mut lines = stdin.lock().lines();
  let s = lines.next().unwrap().unwrap();
  let k: usize = lines.next().unwrap().unwrap().parse().unwrap();
  let mut ss: BTreeSet<&str> = BTreeSet::new();
  for i in 0..s.len() {
    let len = min(5, s.len() - i);
    for j in 0..len {
      ss.insert(&s[i..(i+j+1)]);
    }
  }
  let mut it = ss.iter();
  for _ in 0..(k-1) {
    it.next();
  }
  println!("{}", it.next().unwrap());
}