#include <bits/stdc++.h>
using namespace std;
string find(unordered_set<string>& bar) {
  queue<string> q;
  string x;
  for (char c = 'a'; c <= 'z'; c++) {
    q.push(x + c);
  }
  while (!q.empty()) {
    string s = q.front();
    q.pop();
    if (bar.find(s) == bar.end()) return s;
    for (char c = 'a'; c <= 'z'; c++) {
      q.push(s + c);
    }
  }
  return "";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<string> titles(n);
  for (int i = 0; i < n; i++) cin >> titles[i];
  unordered_set<string> bar;
  for (string title : titles) {
    for (int i = 0; i < title.size(); i++) {
      for (int j = i; j < title.size(); j++) {
        bar.insert(title.substr(i, j - i + 1));
      }
    }
  }
  cout << find(bar) << endl;
  return 0;
}
