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)

//C:\Program Files(x86)\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\x86_64-w64-mingw32\bits\stdc++.h
#include<algorithm>
#include<bitset>
#include<cctype>
#include<cstring>
#include<cstdlib>
#include<deque>
#include<fstream>
#include<functional>
#include<iomanip>
#include<iostream>
#include<limits>
#include<list>
#include<map>
#include<math.h>
#include<queue>
#include<set>
#include<stack>
#include<stdio.h>
#include<sstream>
#include<string>
#include<time.h>
#include<utility>
#include<vector>

using namespace std;

int k;
int cnt[256];
string s;
set<string> st;

int main(){
	cin>>s>>k;
	for(int i=0;i<s.size();i++){
		string str="";
		for(int j=i;j<s.size();j++){
			str+=s[j];
			if(st.find(str)!=st.end()) continue;
			st.insert(str);
			if(st.size()>k){
				set<string>::iterator it=st.begin();
				for(int i=0;i<k;i++,it++);
				st.erase(it,st.end());
			}
		}
	}
	set<string>::iterator it=st.begin();
	for(int i=0;i<k-1;i++,it++);
	cout<<(*it)<<endl;
	return 0;
}