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>
char titles[30][21];
int n;
int checker(char *new_title) {
  int i, j, k;
  for (j = 0; j < n; j++) {
    for (i = 0; titles[j][i] != '\0' && titles[j][i] != new_title[0]; i++)
      ;
    if (titles[j][i] != '\0') {
      for (k = 1; titles[j][k + i] != '\0' && new_title[k] == titles[j][k + i];
           k++)
        ;
      if (new_title[k] == '\0') {
        return 0;
      }
    }
  }
  return 1;
}
int main() {
  scanf("%d", &n);
  int i;
  for (i = 0; i < n; i++) {
    scanf("%s", titles[i]);
  }
  char new_title[21];
  new_title[0] = 'a';
  new_title[1] = '\0';
  int mark = 1;
  while (checker(new_title) == 0) {
    if (new_title[mark - 1] == 'z') {
      new_title[mark - 1] = 'a';
      new_title[mark] = 'a';
      new_title[++mark] = '\0';
    } else {
      new_title[mark - 1] = new_title[mark - 1] + 1;
    }
  }
  printf("%s\n", new_title);
  return 0;
}
