#include <bits/stdc++.h>
using namespace std;
int main() {
  string ans = "";
  string temp = "a";
  string word[35];
  int n;
  bool found;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> word[i];
  }
  for (int i = 0; i < 26; i++) {
    temp[0] = 'a' + i;
    found = false;
    for (int j = 0; j < n; j++) {
      if (word[j].find(temp) < 1000000) {
        found = true;
        break;
      }
    }
    if (found == false) {
      ans = temp;
      break;
    }
  }
  if (ans == "") {
    temp = "aa";
    for (int i = 0; i < 26; i++) {
      for (int k = 0; k < 26; k++) {
        temp[0] = 'a' + i;
        temp[1] = 'a' + k;
        found = false;
        for (int j = 0; j < n; j++) {
          if (word[j].find(temp) < 1000000) {
            found = true;
            break;
          }
        }
        if (found == false) {
          ans = temp;
          cout << ans << endl;
          return 0;
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
