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#include <bits/stdc++.h>
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (((a * b) / gcd(a, b))); }
int phi(int n) {
  int result = n;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      while (n % i == 0) n /= i;
      result -= result / i;
    }
  }
  if (n > 1) result -= result / n;
  return result;
}
int catalan(unsigned int n) {
  int c[100];
  c[0] = c[1] = 1;
  for (int i = (int)(2); i < (int)(n); ++i) {
    for (int k = (int)(0); k < (int)(i); ++k) c[i] += c[k] * c[i - 1 - k];
  }
  return c[n];
}
using namespace std;
int n;
string v[200010];
string s;
bool find(string s) {
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < v[i].length(); j++)
      for (int k = j; k < v[i].length(); k++) {
        string S = v[i].substr(j, k - j + 1);
        if (S == s) return 1;
      }
  }
  return 0;
}
void solve(int pos) {
  int all = 0, bb = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    s[pos] = i;
    all++;
    if (!find(s)) {
      cout << s << endl;
      exit(0);
    } else
      bb++;
  }
  s.resize(2);
  for (int i = 'a'; i <= 'z'; i++) {
    for (int j = 'a'; j <= 'z'; j++) {
      s[0] = i;
      s[1] = j;
      all++;
      if (!find(s)) {
        cout << s << endl;
        exit(0);
      } else
        bb++;
    }
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> v[i];
  s.resize(1);
  solve(0);
  return 0;
}
