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 2
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 10
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#include <bits/stdc++.h>
using namespace std;
struct cmp {
  bool operator()(string x, string y) {
    int p = x.length(), q = y.length();
    if (p < q)
      return true;
    else if (p == q)
      return x < y;
    else
      return false;
  }
} cmp;
int main() {
  int m;
  cin >> m;
  vector<string> seq;
  vector<string> ans;
  for (int i = 0; i < 26; i++) {
    char ch = (char)(i + 'a');
    string tmp = "";
    tmp += ch;
    ans.push_back(tmp);
  }
  for (int i = 0; i < 26; i++) {
    char ch = (char)(i + 'a');
    string tmp = "";
    tmp += ch;
    for (int j = 0; j < 26; j++) {
      char ch2 = (char)(j + 'a');
      tmp += ch2;
      ans.push_back(tmp);
    }
  }
  for (int i = 0; i < m; i++) {
    string s;
    cin >> s;
    int n = s.length();
    for (int i = 0; i < n; i++)
      for (int j = i; j < i + 2; j++) seq.push_back(s.substr(i, j - i + 1));
  }
  sort(seq.begin(), seq.end(), cmp);
  for (int i = 0; i < ans.size(); i++) {
    if (find(seq.begin(), seq.end(), ans[i]) == seq.end()) {
      cout << seq[i];
      return 0;
    }
  }
  return 0;
}
