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>

char top[6][5000]={};

void sort(char top[6][5000]){
    int i,j;
    int flag=0;
    char tmp[5000];
//  for(i=0;i<6;i++)printf("%d::%s\n",i,top[i]);
    for(i=0;i<5;i++)if(strcmp(top[5],top[i])==0)flag=1;
    if(flag==0){
        for(i=1;i<6;i++){
            for(j=1;j<6;j++){
                if(strcmp(top[j-1], top[j])>0 ||strcmp(top[j-1],"\0")==0){
                    strcpy(tmp, top[j-1]);
                    strcpy(top[j-1], top[j]);
                    strcpy(top[j], tmp);
                }
            }
         }
    }
//  for(i=0;i<6;i++)printf("%d::%s\n",i,top[i]);
}

int main(){
    char s[5000];
    int K;
    char hikaku[5000];
    if(scanf("%s",s)==1);
    if(scanf("%d",&K)==1);
    sort(top);
    for(int i=0;i<strlen(s);i++){
        for(int j=i+1;j<=strlen(s);j++){
            strncpy(hikaku,s+i,j-i);
            //printf("%s\n",hikaku);
            strncpy(top[5],hikaku,j-i);
            sort(top);
            memset(hikaku,'\0',5000);
            memset(top[5],'\0',5000);
        }
    }
    printf("%s\n",top[K-1]);
}