#include <bits/stdc++.h>
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  size_t n;
  std::cin >> n;
  std::unordered_set<std::string> set;
  for (size_t i = 0; i < n; ++i) {
    std::string s;
    std::cin >> s;
    for (char c : s) {
      set.insert(std::string(1, c));
    }
    for (size_t j = 0; j + 1 < s.size(); ++j) {
      set.insert(s.substr(j, 2));
    }
  }
  std::string result;
  for (char c = 'a'; c <= 'z'; ++c) {
    if (set.find(std::string(1, c)) == set.end()) {
      result = c;
      break;
    }
  }
  for (char a = 'a'; a <= 'z' && result.empty(); ++a) {
    for (char b = 'a'; b <= 'z'; ++b) {
      std::string cur(1, a);
      cur += b;
      if (set.find(cur) == set.end()) {
        result = cur;
        break;
      }
    }
  }
  std::cout << result << '\n';
}
