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 2
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#include <bits/stdc++.h>
using namespace std;
string s;
bool used[100], use[1000000], uses[1000010];
int n;
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s;
    int k = s.length();
    for (int i = 0; i < k; i++) {
      used[(int)s[i] - 97] = true;
      if (i < k - 1) {
        use[((int)s[i] - 97) * 26 + (int)s[i + 1] - 97] = true;
      }
      if (i < k - 2) {
        uses[((int)s[i] - 97) * 676 + ((int)s[i + 1] - 97) * 26 +
             ((int)s[i + 2] - 97)] = true;
      }
    }
  }
  for (int i = 0; i < 26; i++) {
    if (!used[i]) {
      cout << (char)(i + 97);
      return 0;
    }
  }
  for (int i = 0; i < 676; i++) {
    if (!use[i]) {
      cout << (char)((i / 26) + 97) << (char)((i % 26) + 97);
      return 0;
    }
  }
  for (int i = 0; i < 1000000; i++) {
    if (!uses[i]) {
      cout << (char)((i / 676) + 97) << (char)(((i % 676) / 26) + 97)
           << (char)((i % 26) + 97);
      return 0;
    }
  }
  return 0;
}
