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;
int n, cnt = 0;
string arr[6336], base = "abcdefghijklmnopqrstuvwxyz", a, now = "a";
bool q(string x, string y) {
  if (x.length() != y.length()) return x.length() < y.length();
  return x < y;
}
string nextperm(string x) {
  int l = x.length();
  if (x[l - 1] != 'z') {
    int tmp = (int)x[l - 1];
    tmp++;
    x[l - 1] = (char)tmp;
  } else {
    x[l - 1] = 'a';
    if (l == 1) {
      x = 'a' + x;
      l++;
    } else {
      char buff[22];
      size_t length = x.copy(buff, l - 1, 0);
      buff[length] = '\0';
      string y = buff;
      y = nextperm(y);
      x.replace(0, l - 1, y);
    }
  }
  return x;
}
int main() {
  scanf("%d\n", &n);
  for (int i = 0; i < n; i++) {
    getline(cin, a);
    for (int j = 0; j < a.length(); j++) {
      for (int k = 1; k <= a.length() - j; k++) {
        char b[22];
        size_t length = a.copy(b, k, j);
        b[length] = '\0';
        arr[cnt++] = b;
      }
    }
  }
  sort(arr, arr + cnt, q);
  for (int i = 0; i < cnt; i++) {
    if (now == arr[i]) {
      now = nextperm(now);
    }
  }
  printf("%s\n", now.c_str());
  return 0;
}
