#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
bool cmp(string &a, string &b) {
  if (a.size() == b.size())
    return a < b;
  else
    return a.size() < b.size();
}
void solveCP311() {
  ll n;
  cin >> n;
  map<string, bool> mp;
  for (ll i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (ll i = 0; i < s.size(); i++) {
      for (ll j = 1; i + j <= s.size(); j++) {
        string temp = s.substr(i, j);
        mp[temp] = 1;
      }
    }
  }
  string temp = "";
  vector<string> all;
  for (char i = 'a'; i <= 'z'; i++) {
    all.push_back(temp + i);
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = 'a'; j <= 'z'; j++) {
      all.push_back(temp + i + j);
    }
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = 'a'; j <= 'z'; j++) {
      for (char k = 'a'; k <= 'z'; k++) {
        all.push_back(temp + i + j);
      }
    }
  }
  sort(all.begin(), all.end(), cmp);
  for (auto i : all) {
    if (!mp[i]) {
      cout << i;
      return;
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll t = 1;
  while (t--) {
    solveCP311();
  }
  return 0;
}
