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 30
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 2
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 1
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 10
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 1
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#include <bits/stdc++.h>
using ll = long long;
using namespace std;
int getint() {
  int ans(0);
  char w = getchar();
  while (w < '0' || '9' < w) w = getchar();
  while ('0' <= w && w <= '9') {
    ans = ans * 10 + w - '0';
    w = getchar();
  }
  return ans;
}
unordered_map<string, bool> hsh;
int main() {
  int n = getint();
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 1; j <= s.length(); j++)
      for (int i = 0; i + j <= s.length(); i++)
        hsh[string(s.begin() + i, s.begin() + i + j)] = true;
  }
  for (int i = 0; i < 26; i++)
    if (!hsh[string(1, 'a' + i)]) {
      cout << string(1, 'a' + i);
      return 0;
    }
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < 26; j++)
      if (!hsh[string(1, 'a' + i) + string(1, 'a' + j)]) {
        cout << string(1, 'a' + i) << string(1, 'a' + j);
        return 0;
      }
  return 0;
}
