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;
int main() {
  int n;
  cin >> n;
  set<string> a1, a2, a3;
  string s;
  for (char i = 'a'; i <= 'z'; i++) {
    s = "";
    s += i;
    a1.insert(s);
  }
  for (char i = 'a'; i <= 'z'; i++)
    for (char j = 'a'; j <= 'z'; j++) {
      s = "";
      s += i;
      s += j;
      a2.insert(s);
    }
  for (char i = 'a'; i <= 'z'; i++)
    for (char j = 'a'; j <= 'z'; j++)
      for (char k = 'a'; k <= 'z'; k++) {
        s = "";
        s += i;
        s += j;
        s += k;
        a3.insert(s);
      }
  for (int i = 0; i < n; ++i) {
    cin >> s;
    for (int i = 0; i < s.size(); i++)
      for (int j = 0; j + i - 1 < (int)s.size(); j++) {
        if (a1.count(s.substr(i, j)) > 0) a1.erase(s.substr(i, j));
        if (a2.count(s.substr(i, j)) > 0) a2.erase(s.substr(i, j));
        if (a3.count(s.substr(i, j)) > 0) a3.erase(s.substr(i, j));
      }
  }
  if (!a1.empty())
    cout << *a1.begin();
  else if (!a2.empty())
    cout << *a2.begin();
  else
    cout << *a3.begin();
}
