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

unsigned long long xor64() {
    static unsigned long long x
        = (unsigned long long)(chrono::duration_cast<chrono::nanoseconds>(
            chrono::high_resolution_clock::now().time_since_epoch()).count())
            * 10150724397891781847ULL;
    x ^= x << 7;
    return x ^= x >> 9;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<string> a(n);
    for(auto &&s : a) cin >> s;
    sort(a.begin(), a.end());
    string ans(n, '?');
    auto f = [&](){
        for(int i = 0; i < n; i++){
            ans[i] = xor64() % 2 + 'a';
        }
    };
    while(!binary_search(a.begin(), a.end(), ans)) f();
    cout << ans << '\n';
}