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 30
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 2
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 1
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#include <bits/stdc++.h>
using namespace std;
int n;
set<string> tab;
string dfs(string start) {
  for (int i = 0; i < 26; i++) {
    start += (char)(i + 97);
    if (tab.find(start) == tab.end()) {
      return start;
    }
    start = start.substr(0, start.size() - 1);
  }
  for (int i = 0; i < 26; i++) {
    string tmp = dfs(start + (char)(i + 97));
    if (tmp != "") return tmp;
  }
  return "";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n;
  for (int i = 0; i < n; i++) {
    string a;
    cin >> a;
    for (int j = 0; j < a.size(); j++) {
      for (int k = j; k < a.size(); k++) {
        string b;
        for (int l = j; l <= k; l++) {
          b += a[l];
        }
        tab.insert(b);
      }
    }
  }
  cout << dfs("") << endl;
}
