#include <bits/stdc++.h>
using namespace std;
int alpha[26];
int alphanum = 26;
int word[26][26];
int main(int argc, char **argv) {
  int n;
  string s;
  int pre;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s;
    if (alpha[s[0] - 'a'] == 0) {
      alpha[s[0] - 'a'] = 1;
      alphanum--;
    }
    pre = s[0] - 'a';
    for (int j = 1; j < s.length(); j++) {
      if (alpha[s[j] - 'a'] == 0) {
        alpha[s[j] - 'a'] = 1;
        alphanum--;
      }
      word[pre][s[j] - 'a'] = 1;
      pre = s[j] - 'a';
    }
  }
  if (alphanum != 0) {
    for (int i = 0; i < 26; i++)
      if (alpha[i] == 0) {
        char c = 'a' + i;
        cout << c << endl;
        break;
      }
  } else {
    for (int i = 0; i < 26; i++) {
      for (int j = 0; j < 26; j++) {
        if (word[i][j] == 0) {
          char c, d;
          c = 'a' + i, d = 'a' + j;
          cout << c << d << endl;
          return 0;
        }
      }
    }
  }
  return 0;
}
