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;
string strs[31];
set<string> ss;
void pop(int len) {
  ss.clear();
  for (int i = 0; i < n; i++)
    for (int j = 0; j <= (int)strs[i].size() - len; j++) {
      ss.insert(strs[i].substr(j, len));
    }
}
void inc(string &s) {
  for (int i = s.size() - 1; i >= 0; i--) {
    if (s[i] == 'z')
      s[i] = 'a';
    else {
      s[i] = s[i] + 1;
      return;
    }
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> strs[i];
  int cnt = 0;
  int num = 1;
  while (1) {
    cnt++;
    pop(cnt);
    num *= 26;
    if (ss.size() == num)
      continue;
    else {
      string ans;
      for (int i = 0; i < cnt; ++i) ans += 'a';
      while (1) {
        bool flag = 0;
        for (set<string>::iterator it = ss.begin(); it != ss.end(); it++)
          if (*it == ans) flag = 1;
        if (!flag) {
          cout << ans << endl;
          return 0;
        }
        inc(ans);
      }
      return 0;
    }
  }
}
