#include <bits/stdc++.h>
using namespace std;
int main() {
  istream& in = cin;
  int n;
  in >> n;
  vector<string> v;
  for (int i = 0; i < n; i++) {
    string str;
    in >> str;
    v.push_back(str);
  }
  set<char> use;
  map<char, set<char> > use2;
  for (int i = 0; i < n; i++) {
    char p, c;
    for (int j = 0; j < v[i].length(); j++) {
      c = v[i][j];
      use.insert(c);
      if (j != 0) {
        use2[p].insert(c);
      }
      p = c;
    }
  }
  if (use.size() != 26) {
    for (char ch = 'a'; ch <= 'z'; ch++) {
      if (use.find(ch) == use.end()) {
        cout << ch << endl;
        break;
      }
    }
  } else {
    bool find = false;
    for (char c1 = 'a'; c1 <= 'z' && !find; c1++) {
      set<char> s = use2[c1];
      for (char c2 = 'a'; c2 <= 'z' && !find; c2++) {
        if (s.find(c2) == s.end()) {
          cout << c1 << c2 << endl;
          find = true;
        }
      }
    }
  }
  return 0;
}
