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
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o
a
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 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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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;
int main() {
  cin.tie(0);
  std::ios::sync_with_stdio(false);
  long long n;
  string s;
  map<string, bool> maap;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s;
    for (int k = 0; k < s.size(); k++) {
      for (int h = k; h < s.size(); h++) {
        maap[s.substr(k, h - k + 1)]++;
      }
    }
  }
  s.clear();
  s = "a";
  while (maap[s]) {
    s[s.size() - 1]++;
    if (s[s.size() - 1] > 'z' && s.size() == 1) {
      s = "aa";
    } else if (s.size() == 2 && s[s.size() - 1] > 'z') {
      s[0]++;
      s[s.size() - 1] = 'a';
      if (s[0] > 'z') s = "aaa";
    }
  }
  cout << s << endl;
  return 0;
}
