// #pragma GCC target("avx")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for(int i = 0; i < (int)n; i++)
#define FOR(n) for(int i = 0; i < (int)n; i++)
#define repi(i,a,b) for(int i = (int)a; i < (int)b; i++)
#define all(x) x.begin(),x.end()
//#define mp make_pair
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define pii pair<int,int>
#define vpii vector<pair<int,int>>

template<typename T>
void chmax(T &a, const T &b) {a = (a > b? a : b);}
template<typename T>
void chmin(T &a, const T &b) {a = (a < b? a : b);}

using ll = long long;
using ld = long double;
using ull = unsigned long long;

const ll INF = numeric_limits<long long>::max() / 2;
const ld pi = 3.1415926535897932384626433832795028;
const ll mod = 998244353;
int dx[] = {1, 0, -1, 0, -1, -1, 1, 1};
int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};

#define int long long

void solve() {
    int n;
    cin >> n;
    vector<string> s(n);
    FOR(n) cin >> s[i];

    const int sub_len = min(12LL, n);
    vector<string> subs;
    FOR(n) subs.push_back(s[i].substr(0, sub_len));

    auto ok = [&](string s) -> bool {
        rep(i, n) if(s == subs[i]) return false;
        return true;
    };
    auto nxt = [&](string s) -> string {
        for(int i = sub_len-1; i >= 0; i--) {
            if(s[i] == 'a') {
                s[i]++;
                for(int j = sub_len-1; j > i; j--) {
                    s[j] = 'a';
                }
                break;
            }
        }
        return s;
    };

    string ans(sub_len, 'a');
    
    while(!ok(ans)) {
        ans = nxt(ans);
    }

    cout << ans << endl;
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}