3
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 1
z
 30
b
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k
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a
c
w
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z
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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
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  int n;
  cin >> n;
  vector<string> v(n);
  for (int(i) = (0); (i) < (n); (i)++) cin >> v[i];
  set<string> S;
  for (int(i) = (0); (i) < (n); (i)++)
    for (int(j) = (0); (j) < (v[i].size()); (j)++)
      for (int(k) = (1); (k) < (3); (k)++) {
        string s = v[i].substr(j, k);
        S.insert(s);
      }
  for (int(a) = ('a'); (a) < ('z' + 1); (a)++) {
    string s;
    s += (char)a;
    if (S.find(s) == S.end()) {
      cout << s << '\n';
      return 0;
    }
  }
  for (int(a) = ('a'); (a) < ('z' + 1); (a)++)
    for (int(b) = ('a'); (b) < ('z' + 1); (b)++) {
      string s;
      s += a;
      s += b;
      if (S.find(s) == S.end()) {
        cout << s << '\n';
        return 0;
      }
    }
  return 0;
}
