#include <bits/stdc++.h>
using namespace std;
bool isSubStr(vector<string>& vs, string& s) {
  for (int i = 0; i < vs.size(); i++) {
    if (vs[i].find(s) != string::npos) return true;
  }
  return false;
}
string int2str(int in) {
  string ret;
  if (in == 0) return "a";
  while (in > 0) {
    if (in % 27 == 0) return "a";
    ret = string(1, char('a' - 1 + in % 27)) + ret;
    in /= 27;
  }
  return ret;
}
string solve(vector<string>& vs) {
  int curr = 0;
  string ans = int2str(curr);
  while (isSubStr(vs, ans)) {
    curr++;
    ans = int2str(curr);
  }
  return ans;
}
int main() {
  int n;
  cin >> n;
  vector<string> vs(n);
  for (int i = 0; i < n; i++) {
    cin >> vs[i];
  }
  string ans = solve(vs);
  cout << ans << endl;
  return 0;
}
