3
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 1
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 30
b
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 5
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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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 5
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 2
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 1
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 30
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 30
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 1
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 e
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 aa
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 ab
 z
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 a

#include <bits/stdc++.h>
using namespace std;
char f(set<char> s) {
  set<char>::iterator it;
  set<char>::iterator tm;
  set<char>::iterator ts;
  if (int(*s.begin()) > 97) {
    return 'a';
  }
  for (it = s.begin(); it != --s.end();) {
    tm = ++it;
    ts = tm;
    if (int(*it) - int(*--ts) > 1) {
      return char(int(*ts) + 1);
    }
  }
}
set<string>::iterator itmo;
int main() {
  ifstream fin("out.txt");
  int N;
  cin >> N;
  string x;
  char k;
  string ans;
  set<char> s;
  set<string> s2;
  for (int i = 0; i < N; i++) {
    getline(cin, x);
    for (int j = 0; j < x.size(); j++) {
      s.insert(x[j]);
    }
    for (int j = 1; j < x.size(); j++) {
      string f = "";
      f += x[j - 1];
      f += x[j];
      s2.insert(f);
    }
  }
  int si = s2.size();
  if (s.size() < 26) {
    cout << f(s) << endl;
  } else {
    string ans = "";
    for (int i = 97; i <= 122; i++) {
      for (int j = 97; j <= 122; j++) {
        string po = "";
        po += char(i);
        po += char(j);
        s2.insert(po);
        if (s2.size() != si) {
          cout << po << endl;
          system("pause");
          return 0;
        }
      }
    }
  }
}
