#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:16777216")
using namespace std;
string arr[35];
bool isSubstring(string s1, string s2, int len) {
  for (int i = 0; i < s1.length() - len + 1; i++) {
    if (s1.substr(i, len) == s2) return true;
  }
  return false;
}
int main() {
  string str = "";
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> arr[i];
  for (int i = 0; i < 26; i++) {
    string s = str + char(97 + i);
    bool found = false;
    for (int j = 0; j < n; j++) {
      if (isSubstring(arr[j], s, 1)) {
        found = true;
        break;
      }
    }
    if (!found) {
      cout << s << endl;
      return 0;
    }
  }
  for (int i = 0; i < 26; i++) {
    string s = str + char(97 + i);
    for (int j = 0; j < 26; j++) {
      string sp = s + char(97 + j);
      bool found = false;
      for (int k = 0; k < n; k++) {
        if (isSubstring(arr[k], sp, 2)) {
          found = true;
          break;
        }
      }
      if (!found) {
        cout << sp << endl;
        return 0;
      }
    }
  }
  return 0;
}
