#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:64000000")
using namespace std;
inline unsigned long long gcd(unsigned long long a, unsigned long long b) {
  if (a < b) {
    a ^= b;
    b ^= a;
    a ^= b;
  };
  return (a > b) ? gcd(a - b, b) : a;
}
inline int abs(int x) { return x > 0 ? x : -x; }
struct Trie {
  bool *exist;
  Trie **t;
  Trie() {
    t = new Trie *[26];
    exist = new bool[26];
    for (register int i = 0; i < 26; ++i) exist[i] = false;
  }
};
void process(string x, Trie *B) {
  Trie *K = B;
  int p = x.length(), c;
  for (int i = 0; i < p; ++i) {
    c = x[i] - 'a';
    if (!K->exist[c]) {
      K->t[c] = new Trie();
      K->exist[c] = true;
    }
    K = K->t[c];
  }
}
bool Done = false;
vector<string> q;
int vec = 0;
void search(Trie *B, string s) {
  if (Done) return;
  for (register int i = 0; i < 26; ++i) q.push_back(s + (char)(i + 'a'));
  int len = s.length(), ind;
  Trie *T = B;
  for (int i = 0; i < len; ++i) {
    ind = s[i] - 'a';
    if (!T->exist[ind]) {
      cout << s;
      Done = true;
      return;
    }
    T = T->t[ind];
  }
  search(B, q[vec++]);
}
void solve() {
  Trie B;
  int n, p;
  cin >> n;
  string x;
  for (register int i = 0; i < n; ++i) {
    cin >> x;
    p = x.length();
    for (int k = 0; k <= p; ++k) process(x.substr(k, p - k), &B);
  }
  search(&B, "");
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
