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
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
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sshgcxsky
gecntpcmoojfwp
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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
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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;
const long long N = 1e5 + 10;
const long long mod = 1e9 + 7;
const long long INF = 1e18;
const long long inf = -1e18;
int n;
string a[40];
bool check(string x) {
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < a[i].size(); j++) {
      int t = j, l = 0;
      while (a[i][t] == x[l] && t < a[i].size() && l < x.size()) {
        t++, l++;
      }
      if (l == x.size()) return 1;
    }
  }
  return 0;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  for (int i = 1; i <= 1; i++) {
    string st = "";
    for (int j = 1; j <= i; j++) {
      st += 'a';
    }
    int kp = st.size() - 1;
    while (true) {
      if (check(st) == 0) {
        cout << st;
        return 0;
      }
      if (st[0] == 'z') break;
      st[kp] = char(int(st[kp] + 1));
      kp--;
      if (kp == -1) kp = st.size() - 1;
    }
  }
  for (int i = int('a'); i <= int('z'); i++) {
    for (int j = int('a'); j <= int('z'); j++) {
      char ch1 = char(i), ch2 = char(j);
      string st;
      st = ch1;
      st += ch2;
      if (check(st) == 0) {
        cout << st;
        return 0;
      }
    }
  }
  return 0;
}
