#include <bits/stdc++.h>
const long long MAXA = 1e6 + 1;
using namespace std;
int main() {
  int n;
  string s;
  bool a[26] = {0}, b[26][26] = {false};
  cin >> n;
  while (n--) {
    cin >> s;
    for (long long j = 0; j < s.length(); j++) a[s[j] - 'a'] = 1;
    for (int j = 1; j < s.length(); j++) b[s[j - 1] - 'a'][s[j] - 'a'] = 1;
  }
  for (long long i = 0; i < 26; i++)
    if (!a[i]) {
      i += 97;
      printf("%c", i);
      return 0;
    }
  for (long long i = 0; i < 26; i++)
    for (long long j = 0; j < 26; j++)
      if (!b[i][j]) {
        i += 97;
        j += 97;
        printf("%c", i);
        printf("%c", j);
        return 0;
      }
}
