#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<string> inp;
  for (int i = 1; i <= n; i++) {
    string a;
    cin >> a;
    inp.push_back(a);
  }
  for (int k = -1; k < 26; k++) {
    string b;
    if (k != -1) b += (k + 'a');
    for (char i = 'a'; i <= 'z'; i++) {
      string a = b;
      a += i;
      int check = 1;
      for (int j = 0; j < n; j++) {
        string str = inp[j];
        int t = str.find(a);
        if (t != -1) {
          check = 0;
          break;
        }
      }
      if (check) {
        cout << a << endl;
        return 0;
      }
    }
  }
  return 0;
}
