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 JavaScript (node.js v5.12) (judge verdict: Time Limit Exceeded)

'use strict'

function main(input) {

    input = input.split('\n')
    let str = input[0]
    let k = ~~input[1]

    let substrings = []
    for (let i = 0; i < str.length; i++) {

        let tmp = '';
        for (let n = i + 1; n <= str.length; n++) {

            tmp = str.substring(i, n)
            substrings.push(tmp)
        }

    }
    //重複を整理
    let substrings2 = substrings.filter((v, i, ary) => {
        return ary.indexOf(v) === i
    })
    substrings = substrings2.sort()
    substrings2 = null

    console.log(substrings[k - 1])
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));