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#include <bits/stdc++.h>
using namespace std;
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-15;
set<string> sub;
bool inc(string &x) {
  int carry = 1;
  for (string::reverse_iterator rit = x.rbegin(); rit != x.rend() && carry;
       ++rit) {
    carry += *rit - 'a';
    *rit = carry % ('z' - 'a') + 'a';
    carry /= 'z' - 'a';
  }
  if (carry) {
    return false;
  }
  return true;
}
int main() {
  int n;
  string name[42];
  cin >> n;
  int max_size = -1;
  for (int i = 0; i < n; i++) {
    cin >> name[i];
    max_size = max(max_size, (int)name[i].size());
  }
  string new_sub;
  string test_name;
  set<string>::iterator next;
  for (int i = 1; i <= max_size; i++) {
    sub.clear();
    for (int j = 0; j < n; j++) {
      for (int k = 0; k <= int(name[j].size()) - i; ++k) {
        new_sub.assign(name[j].begin() + k, name[j].begin() + k + i);
        sub.insert(new_sub);
      }
    }
    for (set<string>::iterator sit = sub.begin(); sit != sub.end(); ++sit) {
      test_name = *sit;
      if (!inc(test_name)) break;
      next = sit;
      ++next;
      if (next == sub.end()) break;
      if (test_name != *next) {
        cout << test_name << endl;
        return 0;
      }
    }
  }
  for (int i = 0; i <= max_size; i++) {
    cout << 'a';
  }
  return 0;
}
