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: Wrong Answer)

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

char ss[25000][6];

int compare_s(const void*a, const void*b){
    int* A = (int*)a;
    int* B = (int*)b;
    return strcmp(ss[*A], ss[*B]);
}

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

    char *ps;
    int ls = strlen(s);

    int index = 0;
    for(int h = 1; h <= 5; h++){
        ps = s;
        for(int i = 0; i < ls-h+1; i++){
            strncpy(ss[index], ps, h);
            ss[index][h] = '\0';
            ps++;
            index++;
        }
    }
        
    int num[25000];
    for(int i = 0; i < index; i++){num[i] = i;}
    
    qsort(num, index, sizeof(int), compare_s);
    
    char tmp[6];
    strcpy(tmp, ss[num[0]]);
    
    int cnt = 1;
    if(index == 1){printf("%s\n", tmp); return 0;}
    
    //for(int i = 0; i < index-1; i++){
  	//分かってないけど i < index に変えて出してみる
    for(int i = 0; i < index; i++){
        if(strcmp(tmp, ss[num[i+1]])==0){
            continue;
        }else{
            cnt++;
            strcpy(tmp, ss[num[i+1]]);
        }
        if(cnt == k){printf("%s\n", ss[num[i+1]]); return 0;}
    }
}