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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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 1
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#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  while (cin >> n) {
    int mat[26][26], i, j;
    bool x[26] = {false};
    for (i = 0; i < 26; i++) {
      for (j = 0; j < 26; j++) mat[i][j] = 0;
    }
    string s;
    while (n--) {
      cin >> s;
      for (i = 1; i < s.size(); i++) {
        mat[s[i] - 'a'][s[i - 1] - 'a'] = 1;
        x[s[i] - 'a'] = true;
        x[s[i - 1] - 'a'] = true;
      }
      if (s.size() == 1) x[s[0] - 'a'] = true;
    }
    for (i = 0; i < 26; i++) {
      if (!x[i]) {
        cout << char(i + 'a') << endl;
        break;
      }
    }
    if (i == 26) {
      for (i = 0; i < 26; i++) {
        for (j = 0; j < 26; i++) {
          if (!(mat[i][j] != 0)) {
            cout << char(j + 'a') << char(i + 'a') << endl;
            break;
          }
        }
        if (j != 26) break;
      }
    }
  }
}
