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
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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
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;
int main() {
  istream& in = cin;
  int n;
  in >> n;
  vector<string> v;
  for (int i = 0; i < n; i++) {
    string str;
    in >> str;
    v.push_back(str);
  }
  set<char> use;
  map<char, set<char> > use2;
  for (int i = 0; i < n; i++) {
    char p, c;
    for (int j = 0; j < v[i].length(); j++) {
      c = v[i][j];
      use.insert(c);
      if (j != 0) {
        use2[p].insert(c);
      }
      p = c;
    }
  }
  if (use.size() != 26) {
    for (char ch = 'a'; ch <= 'z'; ch++) {
      if (use.find(ch) == use.end()) {
        cout << ch << endl;
        break;
      }
    }
  } else {
    bool find = false;
    for (char c1 = 'a'; c1 <= 'z' && !find; c1++) {
      set<char> s = use2[c1];
      for (char c2 = 'a'; c2 <= 'z' && !find; c2++) {
        if (s.find(c2) == s.end()) {
          cout << c1 << c2 << endl;
          find = true;
        }
      }
    }
  }
  return 0;
}
