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>
const double eps = 1e-10;
using namespace std;
using ll = long long;
using ul = unsigned long long;
using PII = pair<int, int>;
const int NN = 2002;
vector<int> gr[NN];
ll n, k;
ll m, s, x, a, b, c, d, e, q;
int answww;
int in[NN], aa[NN], bb[3];
int dd[NN][NN];
const string al = "abcdefghijklmnopqrstuvwxyz";
map<char, int> ft;
string gg[NN];
int main() {
  for (int i = 0; i < al.size(); i++) {
    ft[al[i]] = 0;
  }
  scanf("%d", &n);
  while (n--) {
    cin >> gg[a++];
    for (int i = 0; i < gg[a - 1].size(); i++) {
      ft[gg[a - 1][i]] = 1;
    }
  }
  for (int i = 0; i < al.size(); i++) {
    if (ft[al[i]] == 0) {
      cout << al[i];
      return 0;
    }
  }
  for (char a = 'a'; a <= 'z'; a++) {
    for (char b = 'a'; b <= 'z'; b++) {
      string ff = "" + a + b;
      int l = 0;
      for (int i = 0; i < n; i++) {
        if (gg[i].find(ff, 0) != -1) {
          l = 1;
          break;
        }
      }
      if (l) {
        cout << ff << endl;
        return 0;
      }
    }
  }
  return 0;
}
