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 gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int setbit(int n, int idx) { return n | 1 << idx; }
int countbits(int n) {
  int ret = 0;
  while (n) {
    if (n & 1) ret++;
    n /= 2;
  };
  return ret;
}
int getbit(int n, int idx) { return ((n >> idx) & 1) == 1; }
set<string> S[22];
bool solve(string s, int l) {
  if (l == s.length()) {
    if (!S[l].count(s)) {
      cout << s;
      return 1;
    }
    return 0;
  }
  for (char a = 'a'; a <= 'z'; a++) {
    if (solve(s + a, l)) {
      return 1;
    }
  }
  return 0;
}
int main() {
  int n;
  string s;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s;
    int len = 1;
    S[s.length()].insert(s);
    while (len < s.length()) {
      for (int i = 0; i < s.length() - len + 1; i++) {
        string a = s.substr(i, len);
        S[len].insert(a);
      }
      len++;
    }
  }
  for (int i = 1; i <= 10; i++) {
    if (S[i].size() < pow(26.0, i)) {
      string z = "";
      if (solve(z, i)) {
        return 0;
      }
    }
  }
  return 0;
}
