#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s, g = "", a = "abcdefghijklmnopqrstuvwxyz";
  for (int i = 0; i < n; i++) {
    cin >> s;
    g += s + " ";
  }
  for (int i = 0; i < a.size(); i++) {
    string x = "";
    x += a[i];
    if (g.find(x) == string::npos) {
      cout << x;
      return 0;
    }
  }
  for (int i = 0; i < a.size(); i++) {
    for (int j = 0; j < a.size(); j++) {
      string x = "";
      x += a[i];
      x += a[j];
      if (g.find(x) == string::npos) {
        cout << x;
        return 0;
      }
    }
  }
  return 0;
}
