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
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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 main() {
  int i, j, n, t;
  map<string, int> m;
  scanf("%d", &n);
  string s, l;
  for (int i = (int)0; i < (int)n; ++i) {
    cin >> s;
    set<string> q;
    for (int j = (int)0; j < (int)int(s.size()); ++j) {
      for (int k = (int)j; k < (int)int(s.size()); ++k) {
        l = s.substr(j, k - j + 1);
        q.insert(l);
      }
    }
    for (__typeof(q.begin()) it(q.begin()); it != (q.end()); ++it) m[(*it)]++;
  }
  char ch;
  int flag = 0;
  for (i = 1; i <= 21; i++) {
    s = "";
    for (int j = (int)0; j < (int)i; ++j) s += 'a';
    for (j = i; j >= 1; j--) {
      for (ch = 'a'; ch <= 'z'; ch++) {
        s[j - 1] = ch;
        if (m[s] == 0) {
          flag = 1;
          cout << s << endl;
          break;
        }
      }
      if (flag) break;
    }
    if (flag) break;
  }
  return 0;
}
