#include <bits/stdc++.h>
using namespace std;
long long n, m;
const int maxn = 2 * 1000 * 100 + 100;
string s[50];
int num[30];
int bas = 1;
string itos(void) {
  string ans = "";
  int i = 0;
  while (num[i] > -1) {
    ans += char(num[i] + 'a');
    i++;
  }
  string q = "";
  for (int i = ans.length() - 1; i >= 0; i--) q += ans[i];
  return q;
}
bool is(string S) {
  bool ok = true;
  for (int i = 0; i < n; i++)
    if (s[i].find(S) < s[i].length()) ok = false;
  return ok;
}
int main() {
  fill(num, num + 30, -1);
  num[0] = 0;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> s[i];
  while (n) {
    string res = itos();
    if (is(res) == true) {
      cout << res << endl;
      return 0;
    }
    int i = 0;
    num[i]++;
    while (num[i] >= 26) {
      num[i] = 0;
      i++;
      num[i]++;
    }
  }
}
