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 Perl (v5.18.2) (judge verdict: Runtime Error)

my $s = <STDIN>;
my $K = <STDIN>;
chomp $s;
my $len = length $s;
my $length = min(5, $len);

my %h;
foreach my $l (1..$length) {
        foreach my $i (0..$len-$l) {
                my $k = substr($s, $i, $l);
                $h{$k} = ''
        }
}
my $vv;
my $j = 1;
foreach (sort {$a cmp $b} keys %h) {
        if ($j <= $K) {
                $vv = $_;
        } else {
                last;
        }
        $j++;
}
print $vv;