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 Kotlin (1.0.0) (judge verdict: Time Limit Exceeded)

fun main(args: Array<String>) {
    val s = readInputLine().toCharArray()
    val k = readInputLine().toInt()
    
    val substrings = mutableListOf<String>()
    
    for (i in 0 until s.size) {
        var tmp = s[i].toString()
        
        if (compareAndInsert(tmp, substrings) && substrings.size > k && substrings[k] == tmp) {
            continue
        }
        for (j in i + 1 until s.size) {
            tmp = tmp + s[j].toString()
           if (compareAndInsert(tmp, substrings) && substrings.size > k && substrings[k] == tmp) {
                break
           }
        }
    }
    
    println(substrings[k - 1])
}

fun readInputLine(): String {
    return readLine()!!
}


fun compareAndInsert(s: String, list: MutableList<String>): Boolean {
    if (list.contains(s)) {
        return false
    }
    
    for ((index, elem) in list.withIndex()) {
        if (s < elem) {
            list.add(index, s)
            return true
        }
    }
    
    list.add(s)
    return true
}