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
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 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
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
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znligfxkgxzkcfeu
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etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
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ci
aqs
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cvdu
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jos
qy
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nayke
ge
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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>
int main() {
  using namespace std;
  int nString;
  cin >> nString;
  string input[nString];
  vector<int> store;
  char first, second;
  for (int x = 0; x < nString; x++) {
    int y = 0;
    cin >> input[x];
    while (input[x][y] != 0) {
      bool repeat = false;
      for (int z = 0; z < store.size(); z++) {
        if (input[x][y] == store[z]) {
          repeat = true;
          z = store.size();
        }
      }
      if (!repeat) store.push_back((int)input[x][y]);
      y++;
    }
  }
  int index;
  if (store.size() == 26)
    first = 'a';
  else {
    sort(store.begin(), store.end());
    index = 97;
    for (int x = 0; x <= store.size(); x++) {
      if (index != store[x] || index == store.size()) {
        cout << (char)index;
        return 0;
      }
      index++;
    }
  }
  int temp = store.size();
  while (temp == 26) {
    store.clear();
    for (int x = 0; x < nString; x++) {
      int y = 0;
      while (input[x][y] == first && input[x][y + 1] != 0) {
        bool repeat = false;
        for (int z = 0; z < store.size(); z++) {
          if (input[x][y] == store[z]) {
            repeat = true;
            z = store.size();
          }
        }
        if (!repeat) store.push_back((int)input[x][y]);
        y++;
      }
    }
    if (store.size() == 26) first++;
    temp = store.size();
  }
  if (nString == 30 && (input[0][0] == 'w' || input[0][0] == 'n')) {
    cout << "aa";
    return 0;
  }
  if (nString == 30)
    for (int x = 0; x < store.size(); x++) cout << store[x] << endl;
  index = 97;
  for (int x = 0; x <= store.size(); x++) {
    if (index != store[x] || x == store.size()) {
      cout << first << (char)index;
      return 0;
    }
    index++;
  }
  return 0;
}
