3
rjnflsbpxqivrcdjptj
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 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
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l
y
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a
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 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
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 2
rxscdzkkezud
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 5
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abcdefghijklmnopqrst
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 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
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x
i
ci
aqs
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jos
qy
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ge
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t
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 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 str[33], h, c;
int n;
queue<string> q;
string solve() {
  int i, j;
  for (i = 0; i < 26; i++) {
    c = 'a' + i;
    q.push(c);
  }
  while (!q.empty()) {
    h = q.front();
    q.pop();
    for (i = 0; i < n; i++)
      if (str[i].find(h) < str[i].size()) break;
    if (i == n) return h;
    for (i = 0; i < 26; i++) {
      c = h;
      c += 'a' + i;
      q.push(c);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> str[i];
  cout << solve() << endl;
  return 0;
}
