#include <bits/stdc++.h>
using namespace std;
string str[33];
int n;
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    cin >> str[i];
  }
  bool ok = 0;
  for (int i = 0; i < 26; i++) {
    string temp = "";
    temp = ('a' + i);
    ok = 1;
    for (int j = 0; j < n; j++) {
      if (str[j].find(temp) != -1) {
        ok = 0;
        break;
      }
    }
    if (ok) {
      cout << temp << endl;
      break;
    }
  }
  if (ok == 1) return 0;
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      string temp = "";
      temp = ('a' + i);
      temp += ('a' + j);
      ok = 1;
      for (int k = 0; k < n; k++) {
        if (str[k].find(temp) != -1) {
          ok = 0;
          break;
        }
      }
      if (ok == 1) {
        cout << temp << endl;
        break;
      }
    }
    if (ok == 1) break;
  }
  return 0;
}
