#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double EPS = 1e-11;
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
string next(string s) {
  int cur = (int)s.size() - 1;
  while (cur >= 0 && s[cur] == 'z') cur--;
  if (cur == -1)
    s = "a" + s;
  else
    s[cur]++;
  cur++;
  for (int i = cur; i < s.size(); ++i) {
    s[i] = 'a';
  }
  return s;
}
int main() {
  int n;
  string s;
  set<string> stringstream;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> s;
    for (int j = 0; j < s.size(); ++j) {
      for (int k = j; k < s.size(); ++k) {
        stringstream.insert(s.substr(j, k - j + 1));
      }
    }
  }
  string nx = "a";
  while (stringstream.count(nx)) nx = next(nx);
  cout << nx << endl;
  return 0;
}
