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(void){
    char s[5001];
    char *ps = s;
    int k;
    scanf("%s", s);
    scanf("%d", &k);

    char tmp[5000][6];
    char now[6];
    char ans[24990][6];
    char buffer[6];
    int index = -1;
    int ls = strlen(s);
    char right[10][10];

    for(int h = 1; h <= 5; h++){
        for(int i = 0; i < ls-h+1; i++){
            strncpy(tmp[i], ps, h);
            tmp[i][h] = '\0';
            ps++;
        }
        
        for(int i = 0; i < ls-h+1; i++){
            index++;
            strcpy(ans[index], tmp[i]);
        }
            
        ps = s;
    }

    for(int i = 0; i < index; i++){
        for(int j = i+1; j < index+1; j++){
            if(strcmp(ans[i], ans[j]) > 0){
                strcpy(buffer, ans[i]);
                strcpy(ans[i], ans[j]);
                strcpy(ans[j], buffer);
            }
        }
    }
    
    int x = 0;
    int cur = 0;
    while(cur<k){
        if(strcmp(now, ans[x]) !=0 ){
            strcpy(right[cur], ans[x]);
            strcpy(now, ans[x]);
            cur++;
        }
        x++;
    }
    
    printf("%s\n", right[k-1]);

    return 0;  
}