#include <bits/stdc++.h>
using namespace std;
int main() {
  int num;
  while (cin >> num) {
    int i, k;
    vector<string> table;
    for (i = 0; i < num; i++) {
      string stmp;
      cin >> stmp;
      table.push_back(stmp);
    }
    string ans = "a";
    while (1) {
      int isfind = 0;
      for (i = 0; i < num; i++) {
        for (k = 0; k < table[i].length(); k++) {
          string stmp;
          stmp = table[i].substr(k, ans.length());
          if (stmp == ans) {
            isfind = 1;
            break;
          }
        }
      }
      if (!isfind) {
        break;
      } else {
        if (ans[ans.length() - 1] != 'z') {
          ans[ans.length() - 1]++;
        } else {
          int index = ans.length() - 1;
          while (ans[index] == 'z') {
            index--;
          }
          if (index < 0) {
            for (i = 0; i < ans.length(); i++) {
              ans[i] = 'a';
            }
            ans += 'a';
          } else {
            ans[index]++;
            for (i = index + 1; i < ans.length(); i++) {
              ans[i] = 'a';
            }
          }
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}
