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;
const int OO = 2e9;
const int MOD = 1e9 + 7;
char s[31][21], res[22] = "`a```````````````````";
int n, l;
bool ok = false, eq;
int main() {
  int i, j, k;
  freopen("", "r", stdin);
  scanf("%i", &n);
  for (i = 1; i <= n; i++) scanf("%s", s[i]);
  l = 1;
  do {
    ok = false;
    for (i = 1; i <= n; i++) {
      for (j = 0; j <= 19 - l; j++) {
        eq = true;
        for (k = 1; k <= l; k++) eq = eq && (res[k] == s[i][j + k - 1]);
        ok = ok || eq;
      }
    }
    if (ok == true) {
      res[1]++;
      for (i = 1; i <= l; i++) {
        if (res[i] > 'z') {
          res[i] = 'a';
          res[i + 1]++;
          l = max(l, i + 1);
        } else
          break;
      }
    }
  } while (ok == true);
  freopen("", "w", stdout);
  for (i = 1; i <= l; i++) printf("%c", res[l - i + 1]);
}
