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 i, n;
  vector<char> v[5];
  string s[200];
  char j, k;
  for (j = 'a'; j <= 'z'; j = j + 1) {
    v[0].push_back(j);
    v[1].push_back('1');
  }
  for (j = 'a'; j <= 'z'; j = j + 1) {
    for (k = 'a'; k <= 'z'; k++) {
      v[0].push_back(j);
      v[1].push_back(k);
    }
  }
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> s[i];
  }
  int st = 0, cnt;
  for (int i = 0; i < 702; i++) {
    cnt = 0;
    for (int t = 0; t < n; t++) {
      int cn = 0;
      for (int p = 0; p < s[t].size(); p++) {
        if (i <= 25) {
          if (v[0][i] == s[t][p]) {
            cn = 1;
          }
        } else {
          if (v[0][i] == s[t][p] && p != s[t].size() - 1 &&
              v[1][i] == s[t][p + 1]) {
            cn = 1;
          }
        }
      }
      if (!cn) {
        cnt++;
      }
    }
    if (cnt == n) {
      cout << v[0][i];
      if (v[1][i] != '1') cout << v[1][i];
      return 0;
    }
  }
}
