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::*;
use std::str::FromStr;
use std::collections::HashSet;

fn main(){
    let s = read::<String>();
    let k = read::<usize>();
    let slen = s.len();
    let mut a = HashSet::new();
    for i in 0..slen as usize{
        for j in 0..k{
            if i+j>=slen{break;}
            //println!("{}",&s[i..i+j+1]);
            a.insert(&s[i..i+j+1]);
        }
    }
    //println!("{:?}",a);
    let mut b = vec![];
    for str in a{
        b.push(str);
    }
    b.sort();
    println!("{}",b[k-1]);
}
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin.bytes().map(|c| c.expect("Error") as char) 
        .skip_while(|c| c.is_whitespace()).take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("Error")
}