CF 170（div2）B

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本文深入探讨了AI音视频处理技术，特别是视频分割与语义识别的应用，介绍了如何利用深度学习算法实现高效准确的视频分析。

Coming up with a new problem isn't as easy as many people think. Sometimes it is hard enough to name it. We'll consider a title original if it doesn't occur as a substring in any titles of recent Codeforces problems.

You've got the titles of n last problems — the strings, consisting of lowercase English letters. Your task is to find the shortest original title for the new problem. If there are multiple such titles, choose the lexicographically minimum one. Note, that title of the problem can't be an empty string.

A substring s[l... r] (1 ≤ l ≤ r ≤ |s|) of string s = s₁s₂... s_|s| (where |s| is the length of string s) is string s_ls_l + 1... s_r.

String x = x₁x₂... x_p is lexicographically smaller than string y = y₁y₂... y_q, if either p < q and x₁ = y₁, x₂ = y₂, ... , x_p = y_p, or there exists such number r (r < p, r < q), that x₁ = y₁, x₂ = y₂, ... , x_r = y_r and x_r + 1 < y_r + 1. The string characters are compared by their ASCII codes.

Input

The first line contains integer n (1 ≤ n ≤ 30) — the number of titles you've got to consider. Then follow n problem titles, one per line. Each title only consists of lowercase English letters (specifically, it doesn't contain any spaces) and has the length from 1 to 20, inclusive.

Output

Print a string, consisting of lowercase English letters — the lexicographically minimum shortest original title.

Sample test(s)

Input

5
threehorses
goodsubstrings
secret
primematrix
beautifulyear

Output

Input

4
aa
bdefghijklmn
opqrstuvwxyz
c

Output

ab

Note

In the first sample the first 9 letters of the English alphabet (a, b, c, d, e, f, g, h, i) occur in the problem titles, so the answer is letter j.

In the second sample the titles contain 26 English letters, so the shortest original title cannot have length 1. Title aa occurs as a substring in the first title.

没看出来串的长度最多为2，傻b了。26^2 > 600。顺便膜拜大湿一记，随便就yy出来了d题，卧槽。。。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <string>
#include <map>
using namespace std;
const int maxn = 100 + 5;
const int INF = 1000000000;

string str;
map<string,int> M;

int main(){
    int n;
    while(cin >> n){
        M.clear();
        for(int k = 0;k < n;k++){
            cin >> str;
            int len = str.length();
            for(int i = 0;i < len;i++){
                for(int j = 1;i+j <= len;j++){
                    string s(str,i,j);
                    M[s] = 1;
                }
            }
        }
        char ss[3];
        int tag = 0;
        for(char i = 'a';i <= 'z';i++){
            ss[0] = i;ss[1] = '\0';
            string s(ss);
            if(M[s] == 0){
                cout << s << endl;
                tag = 1;
                break;
            }
        }
        if(tag == 1) continue;
        for(char i = 'a';i <= 'z';i++){
            ss[0] = i;
            if(tag == 1) break;
            for(char j = 'a';j <= 'z';j++){
                ss[1] = j;ss[2] = '\0';
                string s(ss);
                if(M[s] == 0){
                    cout << s << endl;
                    tag = 1;
                    break;
                }
            }
        }
    }
    return 0;
}