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 C++14 (GCC 5.4.1) (judge verdict: Runtime Error)

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

int main()
{
	string s;
	int k;
	cin >> s;
	cin >> k;
	
	vector<string> x;
	for(int i = 0; i < s.size(); i++)
	{
		for(int j = 1; j <= 5; j++)
		{
			if(i + j - 1 < s.size())
			{
				x.push_back(s.substr(i, j));
			}
		}
	}
	sort(x.begin(), x.end());
	int tmp = 0;
	if(k == 1)
	{
		cout << x[0] << endl;
	}
	else
	{
		for(int i = 0; i < x.size(); i++)
		{
			if(x[i] != x[i + 1])
			{
				tmp++;
			}
			
			if(tmp == k)
			{
				cout << x[i] << endl;
				return 0;
			}
		}
	}
	return 0;
}