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#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
set<string> s;
string ans;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  for (long long i = 0; i < n; i++) {
    string x;
    cin >> x;
    for (long long i = 0; i < (int)x.size(); i++)
      for (long long j = i; j < (int)x.size(); j++) {
        s.insert(x.substr(i, j - i + 1));
      }
  }
  int ind = 0;
  while (1) {
    for (long long i = 0; i < 26; i++) {
      ans.clear();
      ans.push_back(char(i + 'a'));
      if (!s.count(ans)) {
        cout << ans << '\n';
        return 0;
      }
    }
    for (long long i = 0; i < 26; i++) {
      for (long long j = 0; j < 26; j++) {
        ans.clear();
        ans.push_back(char(i + 'a'));
        ans.push_back(char(j + 'a'));
        if (!s.count(ans)) {
          cout << ans << '\n';
          return 0;
        }
      }
    }
  }
  return 0;
}
