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>
#pragma comment(linker, "/STACK:16777216")
using namespace std;
string arr[35];
bool isSubstring(string s1, string s2, int len) {
  for (int i = 0; i < s1.length() - len + 1; i++) {
    if (s1.substr(i, len) == s2) return true;
  }
  return false;
}
int main() {
  string str = "";
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> arr[i];
  for (int i = 0; i < 26; i++) {
    string s = str + char(97 + i);
    bool found = false;
    for (int j = 0; j < n; j++) {
      if (isSubstring(arr[j], s, 1)) {
        found = true;
        break;
      }
    }
    if (!found) {
      cout << s << endl;
      return 0;
    }
  }
  for (int i = 0; i < 26; i++) {
    string s = str + char(97 + i);
    for (int j = 0; j < 26; j++) {
      string sp = s + char(97 + j);
      bool found = false;
      for (int k = 0; k < n; k++) {
        if (isSubstring(arr[j], sp, 2)) {
          found = true;
          break;
        }
        if (!found) {
          cout << sp << endl;
          return 0;
        }
      }
    }
  }
  return 0;
}
