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 30
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 2
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 10
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#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const int INF = 1000 * 1000 * 1000;
const int ALPH = 26;
struct Node {
  Node *ch[ALPH];
  Node() { memset(ch, 0, sizeof ch); }
};
Node *root;
void addString(const string &s) {
  if (root == 0) {
    root = new Node();
  }
  Node *pt = root;
  for (int p = 0; p < (int)s.size(); ++p) {
    if (pt->ch[s[p] - 'a'] == 0) {
      pt->ch[s[p] - 'a'] = new Node();
    }
    pt = pt->ch[s[p] - 'a'];
  }
}
string query() {
  Node *pt = root;
  string ans = "";
  for (;;) {
    bool solved = false;
    for (int i = 0; i < 26; ++i)
      if (pt->ch[i] == 0) {
        ans += (char)('a' + i);
        solved = true;
        break;
      }
    if (solved) {
      break;
    }
    ans += 'a';
    pt = pt->ch[0];
  }
  return ans;
}
int main() {
  int N;
  cin >> N;
  for (int i = 0; i < N; ++i) {
    string s;
    cin >> s;
    for (int beg = 0; beg < (int)s.size(); ++beg)
      for (int len = 1; beg + len - 1 < (int)s.size(); ++len) {
        addString(s.substr(beg, len));
      }
  }
  cout << query();
  return 0;
}
