#include <bits/stdc++.h>
using namespace std;
long long int i, j, k;
void solve() {
  long long int n;
  cin >> n;
  string s[n];
  for (i = 0; i < n; i++) {
    cin >> s[i];
  }
  bool flag = true;
  string r = "a";
  int x = 'a';
  char c = 'a';
  while (flag) {
    bool fl = true;
    for (i = 0; i < n; i++) {
      size_t found = s[i].find(r);
      if (found != string::npos) {
        fl = false;
        break;
      }
    }
    if (fl == true) {
      cout << r << '\n';
      flag = false;
      return;
    }
    if (c == 'z') {
      flag = false;
      break;
    }
    x++;
    c = x;
    r.pop_back();
    r.push_back(c);
  }
  r = "aa";
  x = 'a';
  int y = 'a';
  c = 'a';
  char d = 'a';
  flag = true;
  while (flag) {
    bool fl = true;
    for (i = 0; i < n; i++) {
      size_t found = s[i].find(r);
      if (found != string::npos) {
        fl = false;
        break;
      }
    }
    if (fl == true) {
      cout << r << '\n';
      flag = false;
      return;
    }
    if (r == "zz") {
      flag = false;
      break;
    }
    if (c == 'z') {
      x++;
      d = x;
      c = 'a';
      y = 'a';
      r.pop_back();
      r.pop_back();
      r.push_back(d);
      r.push_back(c);
    } else {
      y++;
      c = y;
      r.pop_back();
      r.push_back(c);
    }
  }
  return;
}
signed main() {
  solve();
  return 0;
}
