#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for(ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for(ll i = (a); i > (b); --i)
#define REP(i, n) for(ll i = 0; i < (n); ++i)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOREACH(x, a) for(auto &(x) : (a))
#define VECCIN(x)                                                              \
    for(auto &youso_ : (x)) cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()

template <typename T = long long> inline T IN() {
    T x;
    cin >> x;
    return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&... tail) {
    cin >> head;
    CIN(move(tail)...);
}
#define CCIN(...)                                                              \
    char __VA_ARGS__;                                                          \
    CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
    double __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
    ll __VA_ARGS__;                                                            \
    CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
    string __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
    {                                                                          \
        FOREACH(x, v) { cout << x << " "; }                                    \
        cout << "\n";                                                          \
    }
template <typename T = string> inline void eputs(T s) {
    cout << s << "\n";
    exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 5e18;
// const double PI = atan(1.0) * 4.0;
const ll dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

// ローリングハッシュ
// 二分探索で LCP を求める機能つき
struct RollingHash {
    static const int base1 = 1007, base2 = 2009;
    static const int mod1 = 1000000007, mod2 = 1000000009;
    vector<long long> hash1, hash2, power1, power2;

    // construct
    RollingHash(const string &S) {
        int n = (int)S.size();
        hash1.assign(n + 1, 0);
        hash2.assign(n + 1, 0);
        power1.assign(n + 1, 1);
        power2.assign(n + 1, 1);
        for(int i = 0; i < n; ++i) {
            hash1[i + 1] = (hash1[i] * base1 + S[i]) % mod1;
            hash2[i + 1] = (hash2[i] * base2 + S[i]) % mod2;
            power1[i + 1] = (power1[i] * base1) % mod1;
            power2[i + 1] = (power2[i] * base2) % mod2;
        }
    }

    // get hash of S[left:right]
    inline pair<long long, long long> get(int l, int r) const {
        long long res1 = hash1[r] - hash1[l] * power1[r - l] % mod1;
        if(res1 < 0) res1 += mod1;
        long long res2 = hash2[r] - hash2[l] * power2[r - l] % mod2;
        if(res2 < 0) res2 += mod2;
        return {res1, res2};
    }

    // get lcp of S[a:] and T[b:]
    inline int getLCP(int a, int b) const {
        int len = min((int)hash1.size() - a, (int)hash1.size() - b);
        int low = 0, high = len;
        while(high - low > 1) {
            int mid = (low + high) >> 1;
            if(get(a, a + mid) != get(b, b + mid))
                high = mid;
            else
                low = mid;
        }
        return low;
    }
};

bool inc(int l, string &s) {
    for(int i = l - 1; i >= 0; --i) {
        if(s[i] != 'z') {
            s[i]++;
            return true;
        }
        s[i] = 'a';
    }
    return false;
}

signed main() {
    LCIN(N);
    set<string> st;
    REP(i, N) {
        SCIN(S);
        REP(j, S.length()) {
            FOR(len, 1, 5) {
                if(j + len > S.length()) continue;
                st.emplace(S.substr(j, len));
            }
        }
    }
    REP(i, 27) REP(j, 27) REP(k, 27) REP(l, 26) {
        string t;
        if(i) t += 'a' - 1 + i;
        if(j) t += 'a' - 1 + j;
        if(k) t += 'a' - 1 + k;
        t += 'a' + l;
        if(!st.count(t)) eputs(t);
    }
}


Measured memory_bytes: 42472