#include <bits/stdc++.h>
using namespace std;
int N;
string s[35];
bool check(string &r) {
  for (int i = (1), _i = (N); i <= _i; i++)
    if (s[i].find(r) < s[i].size()) return false;
  return true;
}
void movenext(string &r) {
  int L = r.size();
  int j = L - 1;
  for (j; j >= 0; j--) {
    if (r[j] != 'z') {
      r[j]++;
      break;
    } else
      r[j] = 'a';
  }
  if (j == -1) r = r + "a";
}
int main() {
  for (; scanf("%d", &N) != EOF;) {
    for (int i = (1), _i = (N); i <= _i; i++) cin >> s[i];
    string ans = "a";
    for (; !check(ans);) {
      movenext(ans);
    }
    cout << ans << endl;
  }
  return 0;
}
