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 30
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 2
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 5
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 2
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 5
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 1
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 10
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 2
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 1
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 1
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#include <bits/stdc++.h>
using namespace std;
int trie[50000][27], nodes = 0;
void update(string s) {
  int l = s.size();
  vector<int> term(1, 0);
  for (int i = 0; i < l; i++) {
    for (int j = 0; j <= i; j++) {
      if (trie[term[j]][s[i] - 'a'] == -1) {
        trie[term[j]][s[i] - 'a'] = ++nodes;
        if (trie[term[j]][26] == -1)
          trie[term[j]][26] = 1;
        else
          trie[term[j]][26]++;
      }
      term[j] = trie[term[j]][s[i] - 'a'];
    }
    term.push_back(0);
  }
}
string ans;
void findans(string pa, int act) {
  if (ans != "") return;
  if (trie[act][26] != 26) {
    for (int i = 0; i < 26; i++)
      if (trie[act][i] == -1) {
        ans = pa + char(i + 'a');
        break;
      }
    return;
  }
  for (int i = 0; i < 26; i++) findans(pa + char(i + 'a'), trie[act][i]);
}
int main() {
  memset(trie, -1, sizeof trie);
  int n;
  cin >> n;
  string l;
  getline(cin, l);
  while (n--) {
    getline(cin, l);
    update(l);
  }
  findans("", 0);
  cout << ans << endl;
}
