#include <bits/stdc++.h>
#include <atcoder/all>

#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

using namespace std;
using namespace atcoder;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repll(i, n) for (long long i = 0; i < (long long)(n); i++)
#define rep2(i, n, m) for (int i = n; i < (int)(m); i++)
#define repll2(i, n, m) for (long long i = n; i < (long long)(m); i++)
#define all(v) v.begin(),v.end()
using ll=long long;
using ld=long double;
using vi=vector<int>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vl=vector<ll>;
using vvl=vector<vl>;
using vvvl=vector<vvl>;
using vld=vector<ld>;
using vvld=vector<vld>;

int dx[8]={1,0,-1,0,1,1,-1,-1};
int dy[8]={0,1,0,-1,1,-1,1,-1};

const double PI = acos(-1);
//const ll MOD=1e9+7;
//const ll MOD=998244353;
const ll INF=(1LL<<60);
const int INF2=(1<<30);
//using mint=modint1000000007;
//using mint=modint998244353;

int main() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int n;cin>>n;
    set<string> st;
    rep(i,n){
        string s;cin>>s;
        st.insert(s);
    }
    rep(i,1<<10){
        string s="";
        rep(j,min(10,n)){
            if(i&(j<<1))s+='a';
            else s+='b';
        }
        rep(j,n-10)s+='a';
        if(st.count(s))continue;
        cout<<s<<endl;
        return 0;
    }
    return 0;
}