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 30
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 2
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 5
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 2
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 1
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 10
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#include <bits/stdc++.h>
using namespace std;
int n;
vector<string> v[50], lan;
string s;
bool temp = false;
void input() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> s;
    lan.push_back(s);
  }
}
bool olagh(string s) {
  for (int i = 0; i < lan.size(); i++)
    if (lan[i].size() >= s.size() && lan[i].find(s) <= lan[i].size())
      return false;
  return true;
}
void meghdar_dehi() {
  char ch = 'a';
  for (int i = 0; i <= 25; i++) {
    ch = 'a' + i;
    string s;
    s += ch;
    v[1].push_back(s);
    if (olagh(s)) {
      cout << ch;
      temp = true;
      break;
    }
  }
}
int main() {
  input();
  meghdar_dehi();
  if (temp)
    n++;
  else {
    char ch;
    for (int i = 1; i <= 20 && !temp; i++) {
      sort(v[i].begin(), v[i].end());
      for (int j = 0; j < v[i].size() && !temp; j++)
        for (int z = 0; z <= 25 && !temp; z++) {
          ch = 'a' + z;
          string s = v[i][j];
          s += ch;
          v[i + 1].push_back(s);
          if (olagh(s)) {
            cout << s;
            temp = true;
            break;
          }
        }
    }
  }
  return 0;
}
