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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>

using namespace std;

#define ll long long
#define ull unsigned long long
#define PI acos(-1.0)
#define eps 1e-12
#define fi first
#define se second
#define MEM(a,b) memset((a),(b),sizeof(a))
#define mod(x) ((x)%MOD)
#define pii pair<int,int>
#define wz cout<<"-----"<<endl;

const int N = 1e5 + 10, INF = 0x3f3f3f3f , MOD = 1e9 + 7;
const int Move[4][2] = {-1,0,1,0,0,1,0,-1};
const int Move_[8][2] = {-1,-1,-1,0,-1,1,0,-1,0,1,1,-1,1,0,1,1};

inline int read(){
    int x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}


void init() {

}


vector<string> res;
int k;
string s;

void go() {
    string t = s;
    sort(t.begin(),t.end());
    char c = t[0];
    for (int i = 0; i < s.size(); i++) {
        if (s[i] == c) {
            int cnt = 0;
            for (int j = i; j < s.size(); j++) {
                if (cnt++ > 100) break;
                string ss = s.substr(i,j - i + 1);
                res.push_back(ss);
            }
        }
    }
    sort(res.begin(),res.end());
    int y = unique(res.begin(), res.end()) - res.begin();
    cout<<res[k - 1]<<endl;
}

int main(){
//    freopen("C:\\Users\\Administrator\\Desktop\\in.txt","r",stdin);
//    freopen("C:\\Users\\Administrator\\Desktop\\out.txt","w",stdout);
    ios_base::sync_with_stdio(0);
    init();
     cin>>s;
     cin>>k;
    if (s.size() > 50) {
        go();return 0;
    }
    for (int i = 0; i < s.size(); i++) {
        for (int j = i; j < s.size(); j++) {
            string t = s.substr(i,j - i + 1);
            res.push_back(t);
        }
    }
    sort(res.begin(),res.end());
    //for (int i = 0; i < res.size(); i++) cout<<res[i]<<' ';cout<<endl;
    int t = unique(res.begin(), res.end()) - res.begin();
    cout<<res[k - 1]<<endl;

    return 0;
}