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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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 1
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#include <bits/stdc++.h>
using namespace std;
set<string> ss;
int main(void) {
  ios_base::sync_with_stdio(0);
  int n;
  cin >> n;
  vector<string> v(n);
  for (int i = 0; i < (n); i++) {
    cin >> v[i];
    for (int j = 0; j < (((int)(v[i]).size())); j++) {
      string s = "";
      for (int k = (j); k < (((int)(v[i]).size())); k++) {
        s.push_back(v[i][k]);
        ss.insert(s);
      }
    }
  }
  v.clear();
  for (int i = 0; i < (26); i++) {
    string t = "";
    t.push_back(i + 'a');
    v.push_back(t);
  }
  for (int i = 0; i < (26); i++) {
    string t = "";
    t.push_back(i + 'a');
    for (int j = 0; j < (26); j++) {
      v.push_back(t + char(j + 'a'));
    }
  }
  for (int i = 0; i < (26); i++) {
    string t = "";
    t.push_back(i + 'a');
    for (int j = 0; j < (26); j++) {
      for (int k = 0; k < (26); k++) {
        v.push_back(t + char(j + 'a') + char(k + 'a'));
      }
    }
  }
  for (int i = 0; i < (((int)(v).size())); i++) {
    if (!ss.count(v[i])) {
      cout << v[i];
      return 0;
    }
  }
  return 0;
}
