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#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")
using namespace std;
const long double Pi = 3.141592653;
const long long mod = 1e9 + 7;
long long INF = 1000000000000000000;
bool cmp(pair<int, int> a, pair<int, int> b) {
  return (a.first < b.first) || (a.first == b.first && a.second > b.second);
}
bool check(string s, string h) {
  bool ok = true;
  for (int i = 0; i < s.length() - 1; i++) {
    if (s[i] == h[0] && s[i + 1] == h[1]) ok = false;
  }
  if (ok)
    return true;
  else
    return false;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    int n;
    cin >> n;
    string s[n];
    int cnt[26] = {0};
    for (int i = 0; i < n; i++) {
      cin >> s[i];
      for (int j = 0; j < s[i].length(); j++) cnt[s[i][j] - 'a']++;
    }
    bool ok1 = false;
    for (int i = 0; i < 26; i++) {
      if (cnt[i] == 0) {
        cout << char(i + 'a');
        ok1 = true;
        break;
      }
    }
    if (!ok1) {
      bool ok = false;
      for (int i = 0; i < 26; i++) {
        for (int j = 0; j < 26; j++) {
          string h;
          int cnt1 = 0;
          h.push_back(char(i + 'a'));
          h.push_back(char(j + 'a'));
          for (int i = 0; i < n; i++) {
            if (check(s[i], h)) ++cnt1;
          }
          if (cnt1 == n) {
            cout << h;
            ok = true;
            break;
          }
        }
        if (ok) break;
      }
    }
  }
}
