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;
struct classcomp {
  bool operator()(const string& a, const string& b) const {
    if (a.size() == b.size()) return a < b;
    return a.size() < b.size();
  }
};
string next(string a) {
  int i = a.size() - 1, e = 1;
  while (i >= 0 && e) {
    a[i]++;
    if (a[i] == 'z' + 1) {
      a[i] = 'a';
    } else
      e = 0;
    i--;
  }
  if (i == -1 && e)
    return "a" + a;
  else
    return a;
}
int main() {
  int u, k, i, n, j;
  string m;
  set<string, classcomp> s;
  set<string>::iterator it, jt;
  scanf("%d", &k);
  for (u = 0; u < k; u++) {
    cin >> m;
    n = m.size();
    for (i = 0; i < n; i++)
      for (j = i + 1; j <= n; j++) s.insert(m.substr(i, j - i));
  }
  if (s.find("a") != s.end()) {
    bool f = 0;
    string r;
    for (it = s.begin(); !f && it != s.end(); it++) {
      jt = it;
      jt++;
      if (jt == s.end()) break;
      if ((r = next(*it)) != *jt) {
        cout << r;
        f = 1;
      }
    }
    if (!f) cout << next(*s.rbegin());
  } else
    putchar('a');
  return 0;
}
