3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
p
n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
w
u
q
l
y
i
o
a
u
 5
splt
ohqykk
xqpz
knojbur
pmfm
 2
hk
obsp
 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
ubn
nzoc
h
wnjwjrf
t
bq
bw
 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
bool check(vector<string> s, string ss) {
  for (int i = 0; i < s.size(); i++) {
    if (s[i].find(ss) != string::npos && ss != "") {
      return true;
    }
  }
  return false;
}
int addLetters(vector<string> ori, string base, int depth) {
  if (base.length() == depth) {
    if (!check(ori, base)) {
      cout << base << endl;
      return -1;
    }
    return 0;
  }
  for (char letter = 'a'; letter <= 'z'; ++letter) {
    stringstream ss;
    ss << letter;
    int ret;
    ret = addLetters(ori, base + ss.str(), depth);
    if (ret == -1) return -1;
  }
}
int main() {
  int n;
  cin >> n;
  vector<int> d(26, 0);
  vector<string> sts(n);
  string alpha = "abcdefghijklmnopqrstuvwxyz";
  string s;
  for (int i = 0; i < n; i++) {
    cin >> s;
    sts[i] = s;
    for (int j = 0; j < s.length(); j++) d[s.at(j) - 'a'] += 1;
  }
  for (int i = 0; i < n; i++) {
    if (d[i] == 0) {
      cout << alpha.at(i) << endl;
      return 0;
    }
  }
  int ret;
  for (int i = 2; i <= 21; i++) {
    ret = addLetters(sts, "", i);
    if (ret == -1) break;
  }
  return 0;
}
