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;
set<string> A;
string result = "aaaaaaaaaaaaaaaaaaaaa";
string b;
bool getComb(int current) {
  if (current == b.length()) {
    if (!A.count(b)) {
      result = b;
      return true;
    } else
      return false;
  }
  for (int c = 'a'; c <= 'z'; c++) {
    b[current] = c;
    if (getComb(current + 1)) {
      return true;
    }
  }
  return false;
}
void getFirst() {
  for (int len = 1; len <= 20; len++) {
    b += ' ';
    if (getComb(0)) return;
  }
}
int main() {
  int n;
  cin >> n;
  for (int t = 0; t < n; t++) {
    string s;
    cin >> s;
    vector<vector<string> > substrings(s.size());
    for (int i = 0; i < s.size(); i++) {
      substrings[i].resize(s.length() - i);
      substrings[0][i] = string("") + s[i];
      A.insert(substrings[0][i]);
    }
    for (int i = 1; i < substrings.size(); i++) {
      for (int j = 0; j < substrings[i].size(); j++) {
        substrings[i][j] = substrings[i - 1][j] + s[i];
        A.insert(substrings[i][j]);
      }
    }
  }
  getFirst();
  cout << result;
  return 0;
}
