278B.cpp

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <queue>
#include <map>
#include <set>
#include <iostream>
#include <sstream>
#include <stack>
#include <cassert>
#include <fstream>
#include <cstring>
#include <numeric>
#include <ctype.h>
#include <stdio.h>
#define MOD 1000000007  
#define exit return 0
#define inf 10000000000
#define exit return 0
#define PS system("pause")
#define S 100007
#define mx(a,b) a>b?a:b
#define R0(i,n) for (i = 0; i < n; i++)
#define R1(i,n) for (i = 1; i <= n; i++)
#define MS(a,x) memset(x, a, sizeof(x));
#define left (v<<1)
#define right (left|1)
#define mid ((tl+tr)>>1)
#define SZ(x) ((int)x.size())
#define MX(a,b)a>b?a:b
const double eps = 1e-10;


using namespace std;
using ll = long long;
using ul = unsigned long long;
using PII = pair<int, int>;

const int NN = 2002;
vector<int>gr[NN];
ll n, k;
ll m, s, x, a, b, c, d, e, q;
int answww;
int in[NN], aa[NN], bb[3];
int dd[NN][NN];
const string al = "abcdefghijklmnopqrstuvwxyz";
map<char, int>ft;
string gg[NN];
int main(){

	scanf("%d", &n);
	int yy = n;
	for (int xx = 1; xx <= n; xx++){
		cin >> gg[xx];
		for (int i = 0; i < gg[xx].size(); i++){
			ft[gg[xx][i]] = 1;
		}
	}

	for (int i = 0; i < al.size(); i++){
		if (ft[al[i]] == 0){
			cout << al[i];
			exit;
		}
	}
	int ttt = 0;
	for (char a = 'a'; a <= 'z'; a++){
		for (char b = 'a'; b <= 'z'; b++){
			string ff = "";
			ff += a;
			ff += b;

			for (int i = 1; i <= n; i++){
				ttt = gg[i].find(ff, 0);
				if (ttt != -1){
					break;
				}
			}
			if (ttt == -1){
				cout << ff << endl;
				exit;
			}
		}
	}
	exit;
}