#include <bits/stdc++.h>
using namespace std;
const long long N = 1e5 + 10;
const long long mod = 1e9 + 7;
const long long INF = 1e18;
const long long inf = -1e18;
int n;
string a[40];
bool check(string x) {
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < a[i].size(); j++) {
      int t = j, l = 0;
      while (a[i][t] == x[l] && t < a[i].size() && l < x.size()) {
        t++, l++;
      }
      if (l == x.size()) return 1;
    }
  }
  return 0;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  for (int i = 1; i <= 1; i++) {
    string st = "";
    for (int j = 1; j <= i; j++) {
      st += 'a';
    }
    int kp = st.size() - 1;
    while (true) {
      if (check(st) == 0) {
        cout << st;
        return 0;
      }
      if (st[0] == 'z') break;
      st[kp] = char(int(st[kp] + 1));
      kp--;
      if (kp == -1) kp = st.size() - 1;
    }
  }
  for (int i = int('a'); i <= int('z'); i++) {
    for (int j = int('a'); j <= int('z'); j++) {
      char ch1 = char(i), ch2 = char(j);
      string st;
      st = ch1;
      st += ch2;
      if (check(st) == 0) {
        cout << st;
        return 0;
      }
    }
  }
  return 0;
}
