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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 5
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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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 a
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 aa
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 a

#include <bits/stdc++.h>
using namespace std;
template <typename T>
void printVector(const T& t) {
  if (t.size() > 0)
    cout << t[0];
  else {
    cout << "\n";
    return;
  }
  for (int i = int(1); i < int(t.size()); i++) {
    cout << " " << t[i];
  }
  cout << "\n";
}
using ll = long long;
const int mode = 0;
vector<string> words;
set<string> get_sub(int l) {
  set<string> out;
  for (string word : words) {
    int s = word.size();
    for (int i = 0; i + l <= s; i++) {
      out.insert(word.substr(i, l));
    }
  }
  return out;
}
void solve() {
  int n;
  cin >> n;
  words.resize(n);
  for (int i = int(0); i < int(n); i++) cin >> words[i];
  set<string> s1 = get_sub(1);
  if (s1.size() < 26) {
    for (char c = 'a'; c <= 'z'; c++) {
      if (s1.count(string(1, c)) == 0) {
        cout << c << endl;
        return;
      }
    }
  }
  set<string> s2 = get_sub(2);
  if (s1.size() < 26 * 26) {
    for (char c = 'a'; c <= 'z'; c++) {
      for (char d = 'a'; d <= 'z'; d++) {
        if (s2.count(string(1, c) + string(1, d)) == 0) {
          cout << c << d << endl;
          return;
        }
      }
    }
  }
  set<string> s3 = get_sub(3);
  for (char c = 'a'; c <= 'z'; c++) {
    for (char d = 'a'; d <= 'z'; d++) {
      for (char e = 'a'; e <= 'z'; e++) {
        if (s2.count(string(1, c) + string(1, d) + string(1, e)) == 0) {
          cout << c << d << e << endl;
          return;
        }
      }
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int t;
  if (mode == 0) {
    solve();
  } else {
    cin >> t;
    while (t--) {
      solve();
    }
  }
}
