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
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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
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;
string getNext(string s) {
  int n = s.size();
  string res = s;
  for (int i = n - 1; i >= 0; i--) {
    if (res[i] != 'z') {
      res[i] = res[i] + 1;
      break;
    }
  }
  return res;
}
bool sortStr(string a, string b) {
  if (a.size() != b.size()) return a.size() < b.size();
  return a < b;
}
int main() {
  int n;
  cin >> n;
  string a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  set<string> S;
  for (int i = 0; i < n; i++) {
    for (int len = 1; len <= a[i].size(); len++) {
      string s = "";
      int j;
      for (j = 0; j < len; j++) s += a[i][j];
      S.insert(s);
      for (; j < a[i].size(); j++) {
        s.erase(0, 1);
        s += a[i][j];
        S.insert(s);
      }
    }
  }
  string arr[S.size()];
  set<string>::iterator itr;
  int i = 0;
  for (itr = S.begin(); itr != S.end(); ++itr) {
    arr[i++] = *itr;
  }
  sort(arr, arr + S.size(), sortStr);
  string ans;
  if (arr[0] != "a") {
    ans = "a";
  } else {
    for (int i = 0; i < S.size(); i++) {
      string next = getNext(arr[i]);
      if (i != S.size() - 1 && !S.count(next)) {
        ans = next;
        break;
      }
      if (i == S.size() - 1) ans = next;
    }
  }
  cout << ans << endl;
  return 0;
}
