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#include <bits/stdc++.h>
using namespace std;
template <typename TH>
void _dbg(const char *sdbg, TH h) {
  cerr << sdbg << "=" << h << "\n";
}
template <typename TH, typename... TA>
void _dbg(const char *sdbg, TH h, TA... t) {
  while (*sdbg != ',') {
    cerr << *sdbg++;
  }
  cerr << "=" << h << ",";
  _dbg(sdbg + 1, t...);
}
template <class T>
T gcd(T a, T b) {
  return b ? gcd(b, a % b) : a;
}
template <class T>
T lcm(T a, T b) {
  return a * b / gcd(a, b);
}
vector<string> split(string s) {
  istringstream buf(s);
  istream_iterator<string> beg(buf), end;
  vector<string> list(beg, end);
  return list;
}
bool isvowel(char c) {
  string s = "aeiouAEIOU";
  if (find(s.begin(), s.end(), c) != s.end()) return true;
  return false;
}
vector<long long> sieve(long long n) {
  vector<bool> prime(n + 1, true);
  prime[0] = prime[1] = 0;
  for (int i = 2; i * i <= n; i++) {
    if (prime[i])
      for (int j = i * i; j <= n; j += i) {
        prime[j] = 0;
      }
  }
  vector<long long> p;
  for (int i = 2; i <= prime.size(); i++)
    if (prime[i]) p.push_back(i);
  return p;
}
bool isprime(int n) {
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true && (n != 1);
}
int ncr(long long n, long long r) {
  long long C[r + 1];
  memset(C, 0, sizeof(C));
  C[0] = 1;
  for (long long i = 1; i <= n; i++) {
    for (long long j = min(i, r); j > 0; j--)
      C[j] = (C[j] + C[j - 1]) % 1000000007;
  }
  return C[r];
}
bool special(string s) {
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == ' ') continue;
    if (s[i] == '#')
      return true;
    else
      return false;
  }
  return false;
}
vector<long long> ans;
bool cmp(string &a, string &b) {
  return (a.size() < b.size()) || (a.size() == b.size() && a < b);
}
void solve() {
  set<string> s;
  for (int i = 0; i < 26; i++) {
    string a;
    a.push_back('a' + i);
    s.insert(a);
    for (int j = 0; j < 26; j++) {
      string b = a;
      b.push_back('a' + j);
      s.insert(b);
      for (int k = 0; k < 26; k++) {
        string c = b;
        c.push_back('a' + k);
        s.insert(c);
      }
    }
  }
  int n;
  cin >> n;
  vector<string> v(n);
  vector<string> check(s.begin(), s.end());
  sort(check.begin(), check.end(), cmp);
  for (int i = 0; i < n; i++) cin >> v[i];
  for (auto &x : check) {
    int count = 0;
    for (int i = 0; i < n; i++) {
      if (v[i].find(x) == string::npos)
        count++;
      else
        break;
    }
    if (count == n) {
      cout << x;
      return;
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  for (int i = 1; i <= t; i++) {
    solve();
  }
  return 0;
}
