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)

fn main() {
    proconio::input! {
        s: proconio::marker::Bytes,
        k: usize,
    }
    let n=s.len();
    let mut t=std::collections::BTreeSet::new();
    let mut w=s.clone();
    w.sort();
    w.dedup();
    for c in w {
        for i in 0..n {
            if s[i]==c {
                for j in i+1..std::cmp::min(i+k,n)+1 {
                    t.insert(&s[i..j]);
                }
            }
        }
        if t.len()>=k {
            break;
        }
    }
    let mut r: Vec<u8>=vec![];
    for &c in t.iter().skip(k-1).next().unwrap().iter() {
        r.push(c);
    }
    println!("{}",String::from_utf8(r).unwrap());
}