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#include <bits/stdc++.h>
using namespace std;
void debug_out() { cerr << endl; }
void clock_out() {
  cerr << "\nTime Elapsed : " << 1.0 * clock() / CLOCKS_PER_SEC << " s\n";
}
void fileio() {
  freopen("/home/dwai/Desktop/cp/input.txt", "r", stdin);
  freopen("/home/dwai/Desktop/cp/output.txt", "w", stdout);
  freopen("/home/dwai/Desktop/cp/debug.txt", "w", stderr);
}
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << H;
  debug_out(T...);
}
int IT_MAX = 1 << 20;
const long long MOD = 1000000007;
const int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3f;
const double PI = acos(-1);
const double ERR = 1e-10;
int main() {
  42;
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  srand(time(NULL));
  long long n;
  cin >> n;
  vector<string> v(n);
  long long i, j;
  for (i = 0; i < n; i++) cin >> v[i];
  map<string, long long> m;
  for (i = 0; i < n; i++) {
    long long j;
    for (j = 0; j < v[i].length() - 1; j++) {
      m[v[i].substr(j, 2)]++;
      m[v[i].substr(j, 1)]++;
    }
    m[v[i].substr(j, 1)]++;
  }
  for (i = 0; i < 26; i++) {
    string res = "";
    res += ('a' + i);
    if (m[res] == 0) return cout << res, 0;
  }
  for (i = 0; i < 26; i++) {
    for (j = 0; j < 26; j++) {
      string res = "";
      res += ('a' + i);
      res += ('a' + j);
      if (m[res] == 0) return cout << res, 0;
    }
  }
  return 0;
}
