#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<string> arr(n);
  for (int i = 0; i < n; ++i) cin >> arr[i];
  int k = 0;
  bool bad = false;
  string str = "a";
  while (1) {
    for (int i = 0; i < n; ++i) {
      if (arr[i].find(str) != string::npos) {
        bad = true;
        break;
      }
    }
    if (bad) {
      k = str.size() - 1;
      while (k >= 0 && str[k] == 'z') str[k] = 'a', --k;
      if (k == -1)
        str += "a";
      else
        ++str[k];
      bad = false;
    } else
      break;
  }
  cout << str;
  return 0;
}
