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)


// Problem : C - K-th Substring
// Contest : AtCoder Regular Contest 097
// URL : https://atcoder.jp/contests/arc097/tasks/arc097_a
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#include "bits/stdc++.h"
using ll = long long;
using namespace std;
#define size(x) (int)x.size()

const int N = 1e6 + 1, mod = 1e9 + 7;

string s;
int k;
set<string> ms;

int main() { cin.tie(0)->sync_with_stdio(0);
	cin >> s >> k;
	for (int j = 1; j <= min(k, size(s)); j++) {
		for (int i = 0; i <= size(s) - j; i++) {
			ms.insert(s.substr(i, j));
		}
	}
	for (int t = 1; t < k; t++) {
		ms.erase(ms.begin());
	}
	cout << *ms.begin();
}