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>
#include <math.h>
int main(void){
    char c[5100],tmp[7],s[30000][7];
    int k;
    scanf ("%s%d",c,&k);
    strncpy(tmp,c,1);
    tmp[strlen(tmp)]='\0';
    int count=0;
    for (int i=1; i<k+1 && i<strlen(c)+1; i++){
        for(int j=0; j<strlen(c)+1-i; j++){
            strncpy(s[count],c+j,i);
            s[count][strlen(s[count])]='\0';
            count++ ;
        }
    }
     for (int i=0; i<count; i++){
        for (int j=i+1; j<count; j++){
            if (strcmp(s[i],s[j])>0){
                strcpy(tmp,s[i]);
                tmp[strlen(s[i])]='\0';
                strcpy(s[i],s[j]);
                s[i][strlen(s[j])]='\0';
                strcpy(s[j],tmp);
                s[j][strlen(tmp)]='\0';
            }
        }
    }
    int countans=1 ;
    for (int i=0; i<count; i++){
        if(strcmp(s[i],s[i-1])!=0 && i!=0){
            countans++ ; 
        }
        if (countans==k){
            printf ("%s\n",s[i]);
            exit(0);
        }
    }
    return 0 ;
}