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: Accepted)

#include <iostream>
#include <string>
#include <map>
using namespace std;
int main(int argc, char* argv[])
{
	int K;
	string s;
	cin>>s;
	cin>>K;
	map<string,int> myMap;
	map<string,int>::iterator mit;
	int i,j;
	int sLen=s.length();
	int sLen1=min(K,sLen);
	for (i=0;i<sLen;i++){
		for (j=1;j<=sLen1;j++){
			string subS=s.substr(i,j);
			mit=myMap.find(subS);
			if (mit==myMap.end()){
				myMap[subS]=1;
			}
		}

	}
	int k=0;
	for (mit=myMap.begin();mit!=myMap.end();mit++){
		k++;
		if (k==K){
			cout<<(*mit).first<<endl;
			return 0;
		}
	}
	return 0;
}