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
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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
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kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
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qwghrkqwkobrgevsjrk
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
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doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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cvdu
a
jos
qy
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nayke
ge
s
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qwad
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ubn
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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;
vector<string> vt;
string ss[50];
int main() {
  vt.clear();
  string tmp;
  char c, cc;
  for (int i = 0; i < 26; i++) {
    c = i + 'a';
    tmp = c;
    vt.push_back(tmp);
  }
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      c = i + 'a';
      cc = j + 'a';
      tmp = c;
      tmp += cc;
      vt.push_back(tmp);
    }
  }
  int sz = vt.size();
  int n;
  while (cin >> n) {
    for (int i = 0; i < n; i++) cin >> ss[i];
    int f;
    for (int i = 0; i < sz; i++) {
      f = i;
      bool flag = true;
      for (int j = 0; j < n; j++) {
        if (ss[j].find(vt[i]) != string::npos) {
          flag = false;
          break;
        }
      }
      if (flag) break;
    }
    cout << vt[f] << endl;
  }
  return 0;
}
