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: Accepted)

    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
     
     
    void copy(char *to,char *from,int n){
    	int i;
    	for(i=0;i<6;i++){
    		if(i<n)	to[i] = from[i];
    		else to[i] = '\0';
    	}
    }
     
     
    int check(char *a,char *b){
    	int i;
    	for(i=0;i<5;i++){
    		if(a[i] < b[i])			return -1;	//a -> b
    		else if (a[i] > b[i])	return 1;	//b -> a
    	}
    	return 0;	//a == b
    }
     
    int int_sort( const void * a , const void * b ) {
      return strcmp(( char * )a , ( char * )b );
    }
     
    int main(void){
    	char s[5001];
    	int K;
    	int i,j;
    	scanf("%s",s);
    	scanf("%d",&K);
    	
    	char kouho[25000][6];
    	int index = 0;
    	
    	
    	for(i=0;s[i] != '\0';i++){
    		for(j=1; s[i+j-1] != '\0' ;j++){
    			if(j>5) break;
    			copy(kouho[index],&s[i],j);
    			index++;
    		}
    	}
    		
    	
    	qsort(kouho,index,sizeof(kouho[0]),int_sort);
     
    	int ex[25000];
    	for(i=0;i<25000;i++){
    		ex[i] = 1;
    	}
    	for(i=0;i<index;i++){
    		if(check(kouho[i],kouho[i+1]) == 0){
    			ex[i+1] = -1;
    		}
    	}
    	
    		
    	int count = 0,flag = 0;
    	i = 0;
    	while(flag <= K){
    		if(ex[i] > 0){
    			count = i;
    			flag++;
    		}
    		i++;
    	}
    	
    	
    	printf("%s",kouho[count-1]);
    	
    	
    	return 0;}