#include <bits/stdc++.h>
using namespace std;
template <class A, class B>
A cvt(B x) {
  stringstream s;
  s << x;
  A r;
  s >> r;
  return r;
}
typedef vector<int> VI;
typedef vector<pair<int, int> > VII;
typedef vector<string> VS;
set<string> s;
string str, ans, tmp;
bool finds(int l, string tmp) {
  if (l == 0) {
    if (s.find(tmp) == s.end()) {
      ans = tmp;
      return true;
    }
    return false;
  }
  int m = tmp.size();
  tmp.push_back('a');
  for (int i = (int)0; i < (int)26; ++i) {
    tmp[m] = 'a' + i;
    if (finds(l - 1, tmp)) return true;
  }
  return false;
}
int main() {
  int n, l;
  scanf("%d", &n);
  for (int i = (int)0; i < (int)n; ++i) {
    cin >> str;
    for (int j = (int)0; j < (int)str.size(); ++j) {
      for (int k = (int)j; k < (int)str.size(); ++k) {
        tmp = "";
        for (int w = (int)j; w < (int)k + 1; ++w) {
          tmp.push_back(str[w]);
        }
        s.insert(tmp);
      }
    }
  }
  l = 1;
  tmp = "";
  while (true) {
    if (finds(l, tmp)) break;
    l++;
  }
  cout << ans << endl;
  return 0;
}
