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 30
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 2
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#include <bits/stdc++.h>
using namespace std;
int a[1000000];
int main() {
  int n;
  while (cin >> n) {
    memset(a, 0, sizeof(a));
    for (int i = 1; i <= n; i++) {
      string s;
      cin >> s;
      for (int j = 0; j < s.size(); j++) a[s[j] - 'a' + 1]++;
      for (int j = 0; j < s.size() - 1; j++)
        a[(s[j] - 'a' + 1) * 26 + (s[j + 1] - 'a' + 1)]++;
      if (s.size() > 1)
        for (int j = 0; j < s.size() - 2; j++)
          a[(s[j] - 'a' + 1) * 26 * 26 + (s[j + 1] - 'a' + 1) * 26 +
            (s[j + 2] - 'a' + 1)]++;
    }
    for (int i = 1; i < 1000000; i++) {
      if (a[i] == 0) {
        int b[5], t = 26 * 26;
        for (int j = 3; j >= 1; j--) {
          b[j] = i / t;
          i %= t;
          t /= 26;
        }
        if (b[3])
          cout << (char)(b[3] - 1 + 'a') << (char)(b[2] - 1 + 'a')
               << (char)(b[1] + 'a') << endl;
        else {
          if (b[2])
            cout << (char)(b[2] - 1 + 'a') << (char)(b[1] + 'a') << endl;
          else
            cout << (char)(b[1] + 'a') << endl;
        }
        break;
      }
    }
  }
  return 0;
}
