#include <bits/stdc++.h>
using namespace std;
set<string> st;
queue<string> q;
int n;
string s;
int main() {
  while (cin >> n) {
    st.clear();
    for (int i = 0; i < n; ++i) {
      cin >> s;
      for (int j = 0; j < (int)s.length(); ++j) {
        for (int k = 1; j + k <= (int)s.length(); ++k)
          st.insert(s.substr(j, k));
      }
    }
    while (!q.empty()) q.pop();
    q.push("");
    while (!q.empty()) {
      s = q.front();
      q.pop();
      for (char c = 'a'; c <= 'z'; ++c) {
        string s2 = s + c;
        if (st.find(s2) == st.end()) {
          cout << s2 << endl;
          goto X;
        } else
          q.push(s2);
      }
    }
  X:;
  }
  return 0;
}
