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[1001], st[30];
bool use[27];
int len = 1, flag;
void DFS(int cou) {
  if (flag) return;
  while (1) {
    for (int i = 0; i < 26; i++) {
      st[cou] = i + 'a';
      if (cou + 1 < len)
        DFS(cou + 1);
      else if (!strstr(str, st)) {
        printf("%s\n", st);
        flag = 1;
        return;
      }
      if (flag) return;
    }
    len++;
  }
}
int main() {
  int n;
  while (scanf("%d", &n) != EOF) {
    memset(use, 0, sizeof(use));
    flag = 0;
    int cou = 0;
    for (int i = 0; i < n; i++) {
      char ita[30];
      scanf("%s", ita);
      for (int j = 0; j < strlen(ita); j++) {
        str[cou++] = ita[j];
        use[ita[j] - 'a'] = 1;
      }
      str[cou++] = ' ';
    }
    int pos = -1;
    for (int i = 0; i < 26; i++)
      if (!use[i]) {
        pos = i;
        break;
      }
    if (pos != -1) {
      printf("%c\n", pos + 'a');
      continue;
    }
    len++;
    DFS(0);
  }
  return 0;
}
