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#include <bits/stdc++.h>
using namespace std;
#pragma GCC diagnostic ignored "-Wunused-result"
long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return ((a * b) / gcd(a, b)); }
long long p;
string ans = "";
string a, c = "";
vector<string> x;
int fn(string ans) {
  for (long long i = 0; i < c.size(); i++) {
    string d = c.substr(i, p);
    if (d == ans) return -1;
  }
  return 0;
}
int fn1(string ans) {
  for (long long i = 0; i < x.size(); i++) {
    for (long long j = 0; j < x[i].size(); j++) {
      string d = x[i].substr(j, p);
      if (d == ans) return -1;
    }
  }
  return 0;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long i, j, n, t, k;
  int al[26] = {0};
  cin >> n;
  for ((i) = (0); (i) < (n); ++(i)) {
    cin >> a;
    x.push_back(a);
    c += a;
    for (j = 0; j < a.size(); j++) {
      al[a[j] - 97] = 1;
    }
  }
  int flag = 0;
  for (i = 0; i < 26; i++) {
    if (al[i] == 0) {
      p = 1;
      flag = 1;
      ans += char(i + 97);
      long long t = fn(ans);
      if (t == 0) {
        cout << ans;
        return 0;
      } else {
        ans = "";
        flag = 0;
        continue;
      }
    }
  }
  if (flag == 0) {
    ans = "";
    p = 2;
    for (i = 0; i < 26; i++) {
      for (j = 0; j < 26; j++) {
        ans += char(i + 97);
        ans += char(j + 97);
        long long t = fn1(ans);
        if (t == 0) {
          cout << ans;
          return 0;
        } else {
          ans = "";
          continue;
        }
      }
    }
  }
  return 0;
}
