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 (Clang 3.8.0) (judge verdict: Accepted)

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <stdint.h>
#include <string.h>
#include <math.h>

char orderedList[5][6] = { "{{{{{", "{{{{{", "{{{{{", "{{{{{", "{{{{{" };

static void my_strcpy(char * dst, const char * const src, const uint32_t size) {
	for (uint32_t idx = 0; idx < size; idx++) {
		dst[idx] = src[idx];
	}
	dst[size] = '\0';
}

static void moveDownListEntries(const uint32_t pos) {
	for (uint32_t idx = 4; idx > pos; idx--) {
		strcpy(orderedList[idx], orderedList[idx - 1]);
	}
}

static void insertIntoOrderedList(const char * const pSubstring, const uint32_t substringLength, const uint32_t K) {
	char substring[6];

	my_strcpy(substring, pSubstring, substringLength);

	for (uint32_t idx = 0; idx < K; idx++) {
		int cmpResult = strcmp(substring, orderedList[idx]);
		if (cmpResult < 0) {
			moveDownListEntries(idx);
			strcpy(orderedList[idx], substring);
			break;
		}
		else if (cmpResult == 0) {
			break;
		}
	}
}

int main(void) {
	char s[5001];
	uint32_t K;

	scanf("%s\n%"PRIu32, s, &K);

	const uint32_t stringLength = (uint32_t)strlen(s);

	for (uint32_t substringLength = 1; (substringLength <= K) && (substringLength <= stringLength); substringLength++) {
		for (uint32_t substringBeginPos = 0; substringBeginPos <= stringLength - substringLength; substringBeginPos++) {
			insertIntoOrderedList(&(s[substringBeginPos]), substringLength, K);
		}
	}

	printf("%s", orderedList[K - 1]);

	return 0;
}