3
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 1
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 30
b
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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
 a
 e
 a
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 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
inline void superFastRead(int* a) {
  char c = 0;
  while (c < 33) c = fgetc(stdin);
  *a = 0;
  while (c > 33) {
    *a = *a * 10 + c - '0';
    c = fgetc(stdin);
  }
}
bool allZ(string s) {
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != 'z') return 0;
  }
  return 1;
}
string nx_str(string s) {
  if (allZ(s)) {
    for (int i = 0; i < s.length(); i++) s[i] = 'a';
    s = s + 'a';
    return s;
  }
  for (int i = s.length() - 1; i >= 0; i--) {
    if (s[i] == 'z')
      s[i] = 'a';
    else {
      s[i] = s[i] + 1;
      break;
    }
  }
  return s;
}
struct classcomp {
  bool operator()(const string& lhs, const string& rhs) const {
    if (lhs.length() == rhs.length()) return lhs < rhs;
    return lhs.length() < rhs.length();
  }
};
int main() {
  int n;
  cin >> n;
  string S[n];
  for (int i = 0; i < n; i++) cin >> S[i];
  map<string, bool> A;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < S[i].length(); j++) {
      for (int k = j; k < S[i].length(); k++) {
        A[S[i].substr(j, k - j + 1)] = 1;
      }
    }
  }
  string nxt = "a";
  while (A[nxt]) {
    nxt = nx_str(nxt);
  }
  cout << nxt << endl;
}
