3
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 1
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 30
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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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 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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#include <bits/stdc++.h>
using namespace std;
int main() {
  int kkk;
  ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);
  map<string, int> v;
  int n;
  string s;
  cin >> n;
  while (n--) {
    cin >> s;
    for (int i = 0; i < s.size(); i++) {
      string a;
      a += s[i];
      v[a]++;
      if (i < s.size() - 1) {
        a += s[i + 1];
        v[a]++;
      }
    }
  }
  char c;
  for (int i = 0; i < 26; i++) {
    c = 'a' + i;
    string ss;
    ss += c;
    if (v[ss] == 0) {
      cout << ss;
      return 0;
    }
  }
  for (int i = 0; i < 26; i++) {
    c = 'a' + i;
    string ss;
    ss += c;
    for (int k = 0; k < 26; k++) {
      c = 'a' + k;
      string a = ss;
      a += c;
      if (v[a] == 0) {
        cout << a << endl;
        return 0;
      }
    }
  }
}
