#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  cin >> t;
  string s = "";
  while (t--) {
    string a;
    cin >> a;
    s += a;
    s += "%";
  }
  for (char i = 'a'; i <= 'z'; i++) {
    if (s.find(i) == -1) {
      cout << i;
      return 0;
    }
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = i; j <= 'z'; j++) {
      string temp = "";
      temp += i;
      temp += j;
      if (s.find(temp) == -1) {
        cout << temp;
        return 0;
      }
    }
  }
}
