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;
char str[50][30];
char p[30];
char t[30];
int n;
int solve() {
  for (int i = 0; i < n; i++) {
    if (strstr(str[i], t) != NULL) {
      return 0;
    }
  }
  return 1;
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%s", &str[i]);
  }
  p[0] = 'a' - 1;
  p[1] = 0;
  int len = 1;
  for (int i = 1;; i++) {
    p[0]++;
    int j = 0;
    while (p[j] == 'z' + 1) {
      p[j] = 'a';
      j++;
      if (p[j] == 0) {
        p[j] = 'a';
        p[j + 1] = 0;
        len = j + 1;
      } else {
        p[j]++;
      }
    }
    for (int k = 0; k < len; k++) {
      t[k] = p[len - k - 1];
    }
    t[len] = 0;
    if (solve() == 1) {
      break;
    }
  }
  printf("%s\n", t);
  return 0;
}
