3
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 1
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 30
b
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a
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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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a
 a
 e
 a
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 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, cost;
  cin >> n;
  string s;
  bool hash[702];
  memset(hash, false, sizeof(hash));
  while (n--) {
    cin >> s;
    for (int i = 0; i < s.length(); i++) {
      hash[s[i] - 'a'] = true;
    }
    for (int i = 0; i < s.length() - 1; i++) {
      cost = (s[i + 1] - 'a') + 26 * (s[i] - 'a' + 1);
      hash[cost] = true;
    }
  }
  int p;
  for (p = 0; p < 703; p++) {
    if (hash[p] == false) break;
  }
  string final;
  if (p >= 26) {
    char f = 'a' - 1 + (p / 26);
    char s = 'a' + (p % 26);
    final.push_back(f);
    final.push_back(s);
  } else {
    char f = 'a' + p;
    final.push_back(f);
  }
  cout << final << "\n";
}
