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: Wrong Answer)

function Main(input) {
    input = input.split("\n");
    var string = input[0].toString().toLowerCase();
    var K = parseInt(input[1]);
    var substringCounts = string.length;

    if (substringCounts < 1 || 5000 < substringCounts)
        return;

    if (K < 1 || 5 < K)
        return;

    var stacks = [];
    var _substr = '';
    for (var i = 0; i <= substringCounts; i++) {
        for (var j = i; j <= substringCounts; j++) {
            _substr = string.substring(i, j);
            // if (!stacks.includes(_substr) && _substr !== '')
            //     stacks.push(string.substring(i, j));
        }
    }
    stacks.sort();
    console.log("%s", stacks[K - 1]);
}

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