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;
void next(string &x) {
  for (int i = x.size() - 1; i >= 0; --i) {
    if (x[i] != 'z') {
      x[i] = (char)((int)x[i] + 1);
      for (int j = i + 1; j < x.size(); ++j) {
        x[j] = 'a';
      }
      return;
    }
  }
  x = 'a' + x;
  for (int i = 1; i < x.size(); ++i) {
    x[i] = 'a';
  }
}
int main() {
  int n;
  cin >> n;
  string data[n];
  for (int i = 0; i < n; ++i) {
    cin >> data[i];
  }
  string res = "a";
  bool fl = 0;
  while (!fl) {
    fl = 1;
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < data[i].size() - res.size() + 1; ++j) {
        for (int k = 0; k < res.size(); ++k) {
          if (data[i][j + k] != res[k]) {
            break;
          }
          if (k == res.size() - 1) {
            fl = 0;
          }
        }
      }
    }
    if (fl) {
      break;
    }
    next(res);
  }
  cout << res << endl;
  return 0;
}
