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#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <windows.h>
using namespace std;

string min (string a, string b) {
       for (int i = 0; i < min (a.length (), b.length ()); ++i) {
           if (a[i] < b[i]) return a;
           else if (b[i] < a[i]) return b;
       }
       return a;
}

template <typename T1, typename T2>
bool comp (pair<T1,T2> a, pair <T1,T2> b) {return a.first < b.first;}

int main () {
    int n;
    cin >> n;
    string s[n];
    for (int i = 0; i < n; ++i) {
        cin >> s[i];
    }
    string x = "";
    for (char ch = 'a'; ch <= 'z'; ++ch) {
        bool ok = true;
        for (int i = 0; i < n; ++i) {
            if (!ok) break;
            for (int j = 0; j < s[i].length (); ++j) {
                if (s[i][j] == ch) {
                            ok = false;
                            break;
                }
            }
        }
        if (ok) {
                x += ch;
                break;
        }
    }
    for (char c = 'a'; c <= 'z'; ++c){
        for (char ch = 'a'; ch <= 'z'; ++ch) {
            string q = "";
            q += c;
            q += ch;
            bool ok = true;
            for (int i = 0; i < n; ++i) {
                if (!ok) break;
                for (int j = 0; j < s[i].length () - 1; ++j) {
                    if (string(1, s[i][j]) + string(1,s[i][j + 1])== q) {
                                ok = false;
                                break;
                    }
                }
            }
            if (ok) {
                    if (x == "") return cout << q, 0;
                    else cout << min (x, q), 0;
            }
        }
    }
}
