#include <bits/stdc++.h>
using namespace std;
int a[26];
int main() {
  int n;
  string str = "", s = "";
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    cin >> s;
    str += s;
    str += "$";
  }
  int len = str.size();
  char ch, ch1;
  memset(a, 0, sizeof(a));
  for (int i = 0; i < 26; ++i) {
    ch = 'a' + i;
    if (str.find(ch) == string::npos) {
      cout << ch << endl;
      return 0;
    }
  }
  int flag;
  for (int i = 0; i < 26; ++i) {
    s = "";
    ch = 'a' + i;
    flag = 0;
    for (int j = i; j < 26; ++j) {
      flag = 0;
      s = "";
      ch1 = 'a' + j;
      s += ch;
      s += ch1;
      for (int k = 0; k < len - 1; ++k)
        if (str.substr(k, 2) == s) {
          flag = 1;
          continue;
        }
      if (!flag) {
        cout << s << endl;
        return 0;
      }
    }
  }
  return 0;
}
