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;
vector<string> v;
int n;
string s;
map<string, bool> m;
bool comp(string a, string b) {
  if (a.length() != b.length()) {
    return a.length() < b.length();
  }
  return a[a.length() - 1] < b[b.length() - 1];
}
string sig(string mini) {
  string r = "";
  if (mini[mini.length() - 1] == 'z') {
    for (int i = 0; i <= mini.length(); i++) {
      r += "a";
    }
    return r;
  } else {
    mini[mini.length() - 1]++;
    return mini;
  }
}
int main() {
  scanf("%d", &n);
  string mini = "a";
  for (int i = 0; i < n; i++) {
    cin >> s;
    for (int j = 0; j < s.length(); j++) {
      for (int r = 0; r <= s.length(); r++) {
        if (j + r > s.length()) continue;
        string aux = s.substr(j, r);
        if (!m[aux] && aux.length() > 0) v.push_back(aux);
        m[aux] = 1;
      }
    }
  }
  sort(v.begin(), v.end(), comp);
  string res = "";
  for (int i = 0; i < v.size() - 1; i++) {
    if (v[i + 1] != sig(v[i]) && v[i + 1] != v[i]) {
      res = v[i];
      break;
    }
  }
  if (res != "") {
    cout << sig(res) << endl;
  } else {
    cout << sig(v[v.size() - 1]) << endl;
  }
}
