3
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 1
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 30
b
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k
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 5
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 2
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 5
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abcdefghijklmnopqrst
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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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a
 a
 e
 a
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 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
long a[30], n;
vector<string> sp;
vector<string> v;
long subs(string s) {
  long i, j, k, l, m;
  string ss;
  for (i = 0; i < n; i++) {
    for (j = 0; j <= sp[i].size() - s.size(); j++) {
      if ((j + s.size()) > sp[i].size()) break;
      ss = sp[i].substr(j, s.size());
      if (ss == s) {
        return 1;
      }
    }
  }
  return 0;
}
int main() {
  long i, j, k, l, m, p, t;
  string s, ss;
  char ch;
  scanf("%ld", &n);
  for (i = 0; i < n; i++) {
    cin >> s;
    sp.push_back(s);
    for (j = 0; j < s.size(); j++) {
      a[s[j] - 'a'] = 1;
    }
  }
  for (i = 0; i < 26; i++) {
    if (a[i] == 0) {
      printf("%c\n", i + 'a');
      return 0;
    }
    ch = i + 'a';
    ss = ch;
    v.push_back(ss);
  }
  l = 0;
  for (k = 2; k <= 30; k++) {
    for (i = 0; i < 26; i++) {
      t = v.size();
      for (j = l; j < t; j++) {
        ch = i + 'a';
        s = ch + v[j];
        p = subs(s);
        if (!p) {
          cout << s << endl;
          return 0;
        }
        v.push_back(s);
      }
    }
    l = t;
  }
  return 0;
}
