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 s[13000], a[30];
int n, cnt = 0;
bool compare(string i, string j) {
  if (i.length() == j.length()) {
    for (int k = 0; k < i.length(); ++k)
      if (i[k] < j[k])
        return 1;
      else if (i[k] > j[k])
        return 0;
    return 0;
  } else
    return i.length() < j.length();
}
int main() {
  scanf("%d", &n);
  string t;
  for (int i = 0; i < n; ++i) cin >> a[i];
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < a[i].length(); ++j) {
      t = "";
      for (int k = j; k < a[i].length(); ++k) {
        t = t + a[i][k];
        cnt++;
        s[cnt] = t;
      }
    }
  }
  sort(s + 1, s + 1 + cnt, compare);
  s[0] = 'a' - 1;
  s[cnt + 1] = ".";
  int d;
  for (int i = 0; i <= cnt; ++i) {
    if (s[i] == s[i + 1]) continue;
    d = 0;
    for (int j = 0; j < s[i].length(); ++j) d += s[i][j] == 'z';
    if (d == s[i].length()) {
      for (int j = 0; j < s[i].length(); ++j) s[i][j] = 'a';
      s[i] += 'a';
    } else {
      for (int j = s[i].length() - 1; j >= 0; --j) {
        if (s[i][j] == 'z') continue;
        s[i][j]++;
        break;
      }
    }
    if (s[i] != s[i + 1]) {
      cout << s[i];
      return 0;
    }
  }
  return 0;
}
