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

#define DUMP(x) cerr << #x << "=" << x << endl
#define DUMP2(x, y) cerr<<"("<<#x<<", "<<#y<<") = ("<<x<<", "<<y<<")"<< endl
#define BINARY(x) static_cast<bitset<16> >(x)

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)

#define in_range(x, y, w, h) (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()

typedef long long ll;
const int INF   = 1e9;
const ll  INFLL = 1e18;
typedef pair<int, int> PII;
int dx[4]={0, -1, 1, 0}, dy[4]={-1, 0, 0, 1};

set<string> pass_list;

string solve(string now, int length)
{
    assert(now.size() <= 4);

    if (now.size() == length) {
        return pass_list.count(now) ? "" : now;
    }

    rep(i, 26) {
        string tmp = solve(now + (char)('a' + i), length);
        if (tmp.size() > 0) return tmp;
    }

    return "";
}

int main()
{
    ios::sync_with_stdio(false);

    int N; cin >> N;
    vector<string> S(N);
    rep(i, N) cin >> S[i];

    for (auto s : S) {
        rep(i, s.size()) {
            string tmp;
            rep(j, 4) {
                if (i + j < (int)s.size()) {
                    tmp += s[i + j];
                    pass_list.insert(tmp);
                }
            }
        }
    }

    rep(i, 4) {
        string tmp = solve("", i+1); 
        if (tmp != "") {
            cout << tmp << endl;
            break;
        }
    }
}