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 text_io::*;
use proconio::*;
use std::collections::*;
use itertools::Itertools;
use std::process::exit;
use std::cmp::*;
use num::*;
use num::integer::Roots;
use std::str::FromStr;
use std::io::stdin;

fn main(){
    input! {
    a:String,
    b:usize,
    }
    let mut v=HashSet::new();
    let mut w:Vec<String>=Vec::new();
    let mut a=a.chars().collect_vec();
    for i in 0..a.len(){
        for j in i..min(a.len(),i+5){
            let mut z=v.len();
            let mut g =Vec::new();
            for k in i..=j{
                g.push(a[k])
            }
            let h:String=g.iter().collect();
            let g=g.iter().collect();
            v.insert(h);
            if v.len()!=z{
                w.push(g);
            }
        }
    }
    w.sort();
    println!("{}",w[b-1])
}