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 <bits/stdc++.h>

using namespace std;

#define ll 			long long
#define db			double
#define up(i,j,n)		for (int i = j; i <= n; i++)
#define down(i,j,n)	for (int i = j; i >= n; i--)
#define cadd(a,b)		a = add (a, b)
#define cpop(a,b)		a = pop (a, b)
#define cmul(a,b)		a = mul (a, b)
#define pr			pair<int, int>
#define fi			first
#define se			second
#define SZ(x)		(int)x.size()
#define bin(i)		(1 << (i))
#define Auto(i,node)	for (int i = LINK[node]; i; i = e[i].next)

template<typename T> inline bool cmax(T & x, T y){return y > x ? x = y, true : false;}
template<typename T> inline bool cmin(T & x, T y){return y < x ? x = y, true : false;}

const int MAXN = 5005;
const int MAXM = MAXN * 5;

string ss[MAXN], s;
int K, cnt;

int main(){
	cin >> s >> K;
	int len = s.length();
	up (i, 0, len - 1) up (j, i, min(i + K - 1, len - 1)) 
		ss[++cnt] = s.substr(i, j - i + 1);
	sort(ss + 1, ss + cnt + 1);
	cnt = unique(ss + 1, ss + cnt + 1) - ss - 1;
	cout << ss[K] << endl; 
	return 0;
}