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#include <bits/stdc++.h>
using namespace std;
using namespace std;
bool isprime(int n) {
  if (n == 1) return false;
  for (int i = 2; i <= (int)sqrt((double)n); i++) {
    if (n % i == 0) return false;
  }
  return true;
}
bool is_pal(string str) {
  int l = 0;
  int r = str.length() - 1;
  while (l < r) {
    if (str[l] != str[r]) break;
    l++;
    r--;
  }
  if (l == r)
    return true;
  else
    return false;
}
int diff(string a, string b) {
  int dif = 0;
  for (int i = 0; i < min(a.length(), b.length()); i++) dif += (b[i] - a[i]);
  for (int i = min(a.length(), b.length()) - 1; i < max(a.length(), b.length());
       i++)
    dif += b[i];
  return dif;
}
int main() {
  int n;
  cin >> n;
  string *s = new string[n];
  vector<string> sub;
  int i, j, k;
  bool x[26] = {false};
  for (i = 0; i < n; i++) {
    cin >> s[i];
    for (j = 0; j < s[i].length(); j++) {
      for (k = 0; k < 26; k++) {
        if (s[i][j] == ('a' + k)) {
          x[k] = true;
        }
      }
    }
  }
  for (i = 0; i < 26; i++)
    if (!x[i]) {
      cout << (char)('a' + i);
      return 0;
    }
  int c = 0;
  for (i = 0; i < n; i++) {
    for (j = 0; j < s[i].length(); j++) {
      for (k = 1; k < s[i].length() - j; k++) {
        sub.push_back(s[i].substr(j, k + 1));
      }
    }
  }
  sort(sub.begin(), sub.end());
  for (i = 0; i < sub.size() - 1; i++) {
    if (diff(sub[i], sub[i + 1]) >= 1) {
      cout << sub[i].substr(0, sub[i].length() - 1) +
                  (char)(sub[i][sub[i].length() - 1] + 1);
      return 0;
    }
  }
  return 0;
}
