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 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include<stdio.h>
#include<string.h>

int main() {
	char s[5000]; char buff[5000];
	int K;
	char top[5][5000] = { {"{"},{"{"},{"{"},{"{"},{"{"} };
	scanf("%s", s);
	scanf("%d", &K);
	for (unsigned int i = 1; i <= strlen(s); i++) {
		for (unsigned int j = 0; j <= strlen(s) - i; j++) {
			strncpy(buff, s + j, i);
			buff[i] = '\0';
			for (int k = 0; k <= 4; k++) {
				if (!strcmp(top[k], buff))break;
				else if (strcmp(top[k], buff) > 0) {
					for (int h = 4; h > k; h--) {
						strcpy(top[h], top[h - 1]);
					}
					strncpy(top[k], buff, i);
					break;
				}
			}
		}
	}
	printf("%s\n", top[K - 1]);

	return 0;
}