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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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#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 1, -1, 0};
int dy[] = {1, 0, 0, -1};
long long pw(long long b, long long p) {
  if (!p) return 1;
  long long sq = pw(b, p / 2);
  sq *= sq;
  if (p % 2) sq *= b;
  return sq;
}
const int N = (2 * 1e5) + 10, M = N * 2;
map<string, bool> mp;
string solve() {
  for (char i = 'a'; i <= 'z'; i++) {
    string t;
    t += i;
    if (!mp[t]) return t;
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = 'a'; j <= 'z'; j++) {
      string t;
      t += i;
      t += j;
      if (!mp[t]) return t;
    }
  }
  return "5ara";
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (auto j : s) {
      string t;
      t += j;
      mp[t] = 1;
    }
  }
  cout << solve();
  return 0;
}
