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
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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
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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 MOD = 1e9 + 7;
const long long mx = 2e5;
const int N = 1e6 + 6;
bool ase_naki(string s, string a) {
  for (int n = 0; n < s.size() - 1; n++) {
    if ((s[n] == a[0]) && (s[n + 1] == a[1])) return 0;
  }
  return 1;
}
int main() {
  unordered_map<char, bool> vis;
  string s;
  long long x;
  scanf("%lld", &(x));
  vector<string> vss(x);
  for (int n = 0; n < x; n++) {
    cin >> s;
    vss[n] = s;
    for (auto j : s) vis[j] = 1;
  }
  for (char a = 'a'; a <= 'z'; a++) {
    if (!vis[a]) {
      cout << a;
      return 0;
    }
  }
  string chk = "aa";
  bool hoise = 1;
  while (hoise) {
    hoise = 0;
    for (int n = 0; n < x; n++) {
      if (!ase_naki(vss[n], chk)) {
        hoise = 1;
        chk[1]++;
        break;
      }
    }
    if (!hoise) {
      cout << chk;
      return 0;
    }
  }
  chk = "ba";
  hoise = 1;
  while (hoise) {
    hoise = 0;
    for (int n = 0; n < x; n++) {
      if (!ase_naki(vss[n], chk)) {
        hoise = 1;
        chk[1]++;
        break;
      }
    }
    if (!hoise) {
      cout << chk;
      return 0;
    }
  }
}
