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[35];
map<string, int> a;
string getst(int num) {
  string ans;
  if (num < 26) {
    ans += ('a' + num);
    return ans;
  }
  while (num) {
    ans += (num % 26 + 'a');
    num /= 26;
  }
  reverse((ans).begin(), (ans).end());
  return ans;
}
int main() {
  int m, n, i, j, k;
  ios::sync_with_stdio(false);
  ;
  cin >> m;
  for (i = 1; i <= m; i++) cin >> s[i];
  for (i = 1; i <= m; i++) {
    for (j = 0; j < s[i].length(); j++) {
      for (k = 1; k + j <= s[i].length(); k++) {
        int num = k, tmp = j;
        string b;
        while (num--) {
          b += s[i][tmp];
          tmp++;
        }
        a[b] = 1;
      }
    }
  }
  for (i = 'a'; i <= 'z'; i++) {
    string ans;
    ans += i;
    if (a[ans] == 0) {
      cout << ans << endl;
      return 0;
    }
  }
  for (j = 'a'; j <= 'z'; j++)
    for (i = 'a'; i <= 'z'; i++) {
      string ans;
      ans += j;
      ans += i;
      if (a[ans] == 0) {
        cout << ans << endl;
        return 0;
      }
    }
}
