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
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
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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
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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 MAX = 19000;
int n, cnt;
string a[MAX];
string b[35];
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> b[i];
  for (int i = 0; i < 26; i++) a[cnt++] += 'a' + i;
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < 26; j++) {
      a[cnt] += 'a' + i;
      a[cnt++] += 'a' + j;
    }
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < 26; j++)
      for (int z = 0; z < 26; z++) {
        a[cnt] += 'a' + i;
        a[cnt] += 'a' + j;
        a[cnt++] += 'a' + z;
      }
  for (int i = 0; i < cnt; i++) {
    bool check = false;
    for (int j = 0; j < n; j++) {
      bool help;
      for (int z = 0; z < max(0, (int)(b[j].size() - a[i].size() + 1)); z++) {
        help = true;
        for (int k = 0; k < a[i].size(); k++)
          if (a[i][k] != b[j][z + k]) help = false;
        if (help) break;
      }
      if (help) {
        check = true;
        break;
      }
    }
    if (!check) {
      cout << a[i] << endl;
      break;
    }
  }
  return 0;
}
