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;
string lexico_next(string s) {
  int i, n = s.size();
  for (i = n - 1; i >= 0; i--)
    if (s[i] < 'z') {
      s[i]++;
      for (i++; i < n; i++) s[i] = 'a';
      return s;
    }
  return "";
}
int main() {
  int n, i, m, j, k, z[21] = {0}, q;
  string s, np;
  vector<string> v;
  set<string> st;
  set<string>::iterator it;
  next_permutation(s.begin(), s.begin() + 3);
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> s;
    m = s.size();
    for (j = 1; j <= m; j++)
      for (k = 0; k <= m - j; k++) {
        q = st.size();
        st.insert(s.substr(k, j));
        if (st.size() > q) z[j]++;
      }
  }
  for (q = 26, i = 1; i < 21; i++, q *= 26)
    if (z[i] < q) break;
  np = string(i, 'a');
  for (it = st.begin(); it != st.end(); it++)
    if ((*it).size() == i) {
      if (np < (*it)) {
        cout << np;
        return 0;
      }
      np = lexico_next(*it);
    }
  return 0;
}
