#include <bits/stdc++.h>
using namespace std;
bool comp(const int a, const int b) { return a > b; }
long long binpow(long long a, long long n, int mod) {
  long long res = 1;
  while (n)
    if (n & 1) {
      res *= a;
      res %= mod;
      --n;
    } else {
      a *= a;
      a %= mod;
      n >>= 1;
    }
  if (res == 0) res = mod;
  return res;
}
vector<int> prefix_function(string s) {
  int n = (int)s.length();
  vector<int> pi(n);
  for (int i = 1; i < n; ++i) {
    int j = pi[i - 1];
    while (j > 0 && s[i] != s[j]) j = pi[j - 1];
    if (s[i] == s[j]) ++j;
    pi[i] = j;
  }
  return pi;
}
string ss1, ss2;
int main() {
  vector<int> a = prefix_function("a#aa");
  int n;
  cin >> n;
  string s, str;
  vector<string> v;
  for (int i = 0; i < n; i++) {
    cin >> str;
    v.push_back(str);
  }
  for (int i = 'a'; i <= 'z'; i++) {
    bool bb = true;
    ss1 = i;
    string f = ss1;
    for (int ii = 0; ii < n && bb == true; ii++) {
      vector<int> a = prefix_function(f + "#" + v[ii]);
      for (int jj = f.length() + 1; jj < a.size() && bb == true; jj++) {
        if (a[jj] == f.size()) {
          bb = false;
          break;
        }
      }
    }
    if (bb == true) {
      cout << f;
      return 0;
    }
  }
  for (int i = 'a'; i <= 'z'; i++) {
    for (int j = 'a'; j <= 'z'; j++) {
      bool bb = true;
      ss1 = i;
      ss2 = j;
      string f = ss1.append(ss2);
      for (int ii = 0; ii < n && bb == true; ii++) {
        vector<int> a = prefix_function(f + "#" + v[ii]);
        for (int jj = f.length() + 1; jj < a.size() && bb == true; jj++) {
          if (a[jj] == f.size()) {
            bb = false;
            break;
          }
        }
      }
      if (bb == true) {
        cout << f;
        return 0;
      }
    }
  }
  return 0;
}
