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;
string curr = "a";
string last[31];
int n;
void inc_str() {
  int i, j;
  for (i = curr.size() - 1; i >= 0; i--) {
    if (int(curr[i]) < int('z')) {
      curr[i] = char(int(curr[i]) + 1);
      break;
    } else if (i == curr.size() - 1) {
      int sz = curr.size();
      for (j = 0; j < sz; j++) curr[i] = 'a';
      curr += 'a';
      i = curr.size() - 1;
      break;
    }
  }
  for (i++; i < curr.size(); i++) curr[i] = 'a';
}
bool check() {
  int i, j, h, sz, tmp;
  for (i = 0; i < n; i++) {
    sz = last[i].size();
    for (j = 0, h = 0; j < sz; j++) {
      if (last[i][j] == curr[h]) {
        if (h == 0) tmp = j;
        h++;
      } else {
        if (h != 0) j = tmp;
        h = 0;
      }
      if (h == curr.size()) return true;
    }
  }
  return false;
}
int main() {
  int i;
  cin >> n;
  for (i = 0; i < n; i++) cin >> last[i];
  while (check()) inc_str();
  cout << curr;
  return 0;
}
