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
obsp
 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
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q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
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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>
using namespace std;
map<string, int> ap;
vector<string> val[30];
int sub(string a, string b) {
  int cnt = 0;
  for (int i = 0; i < a.size(); i++) {
    cnt += (b[i] - a[i]);
  }
  return cnt;
}
int main(void) {
  int n, sz = 0;
  cin >> n;
  char s[30];
  string k;
  for (int i = 0; i < n; i++) {
    scanf("%s", s);
    for (int j = 0, len = strlen(s); j < len; j++) {
      k = s[j];
      if (ap[k] == 0) {
        val[0].push_back(k);
        ap[k]++;
      }
      for (int l = j + 1; l < len; l++) {
        k = "";
        for (int z = j; z <= l; z++) k += s[z];
        if (ap[k] == 0) {
          val[l - j].push_back(k);
          ap[k]++;
          sz = max(sz, l - j + 1);
        }
      }
    }
  }
  for (int i = 0; i < sz; i++) {
    sort(val[i].begin(), val[i].end());
    string tmp = "";
    for (int j = 0; j < i + 1; j++) tmp += 'a';
    if (sub(tmp, val[i][0]) > 1) {
      cout << tmp;
      return 0;
    }
    for (int j = 0; j < val[i].size() - 1; j++) {
      if (sub(val[i][j], val[i][j + 1]) > 1) {
        cout << (val[i][j].substr(0, val[i][j].size() - 1));
        cout << char(val[i][j][val[i][j].size() - 1] + 1);
        return 0;
      }
    }
  }
  return 0;
}
