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
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 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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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;
namespace round170 {
namespace B {
inline string next(string a) {
  for (int i = (int)a.size() - 1; i >= 0; --i) {
    if (a[i] == 'z') {
      a[i] = 'a';
      if (i == 0) a = "a" + a;
    } else {
      ++a[i];
      return a;
    }
  }
  return a;
}
inline bool cmp(const string& a, const string& b) {
  if (a.size() == b.size()) return a < b;
  return a.size() < b.size();
}
int solve(istream& in) {
  int n;
  in >> n;
  set<string> q;
  string s;
  string ans;
  for (int i = 0; i < n; ++i) {
    in >> s;
    for (int j = 0; j < (int)s.size(); ++j)
      for (int k = j + 1; k <= (int)s.size(); ++k) {
        string t;
        for (int z = j; z < k; ++z) t += s[z];
        q.insert(t);
      }
  }
  vector<string> v(q.begin(), q.end());
  sort(v.begin(), v.end(), cmp);
  int i = 0;
  s = v[i];
  if (s != "a")
    ans = "a";
  else {
    for (++i; i < (int)v.size(); ++i) {
      s = next(s);
      if (v[i] != s) {
        ans = s;
        break;
      }
    }
    if (ans.empty()) {
      ans = next(v.back());
    }
  }
  cout << ans << endl;
  return 999;
}
}  // namespace B
}  // namespace round170
int main() {
  ios_base::sync_with_stdio(false);
  int status = 0;
  do {
    status = round170::B::solve(cin);
  } while (0);
  return 0;
}
