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 <stdlib.h>
#include <string.h>
char s[5005], ss[22001][22001], sn[3];
int k, fv[200], nr;
int partition (int low, int high)
{
    char pivot[5005];    // pivot
    strcpy(pivot, ss[high]);
    char aux[5005];
    int j;
    int i = (low - 1);  // Index of smaller element

    for (j = low; j <= high- 1; j++)
    {
        // If current element is smaller than or
        // equal to pivot
        if (strcmp(ss[j], pivot) <=0)
        {
            i++;    // increment index of smaller element
            strcpy(aux, ss[i]);
            strcpy(ss[i], ss[j]);
            strcpy(ss[j], aux);
        }
    }
    strcpy(aux, ss[i + 1]);
            strcpy(ss[i + 1], ss[high]);
            strcpy(ss[high], aux);
    return (i + 1);
}
void quickSort(int low, int high)
{
    if (low < high)
    {
        /* pi is partitioning index, arr[p] is now
           at right place */
        int pi = partition(low, high);

        // Separately sort elements before
        // partition and after partition
        quickSort(low, pi - 1);
        quickSort(pi + 1, high);
    }
}
void unificare()
{
    int i, j;
    for(i = 0; i < nr - 1; i ++)
        if(strcmp(ss[i], ss[i + 1]) == 0)
        {
        for(j = i + 1; j < nr - 1; j ++)
            strcpy(ss[j], ss[j + 1]);
        nr --;
        i --;
        }
}
int main()
{
    scanf("%[^\n]%*c", s);
    scanf("%d", &k);
    int i, j;
    for(i = 0; i < strlen(s); i ++)
        if(!fv[(int)s[i]])
        {
            sn[0] = s[i];
            strcpy(ss[nr ++], sn);
            fv[(int)s[i]] ++;
        }
   for(i = 0; i < strlen(s) - 1; i ++)
        for(j = i + 1; (j < strlen(s)) && ((j - i + 1) <= k); j ++)
            if((j - i + 1) <= k)
                strncpy(ss[nr ++], s + i, j - i + 1);
    quickSort(0, nr - 1);
    unificare();
    printf("%s", ss[k - 1]);
    /*for(i = 0; i < nr; i ++)
        printf("%s\n", ss[i]);*/
   return 0;
}