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
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
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
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
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;
string s[35];
map<string, int> a;
string pt(int ans) {
  string b;
  while (ans) {
    b += ('a' - 1 + ans % 26);
    ans /= 26;
  }
  reverse((b).begin(), (b).end());
  return b;
}
int main() {
  int m, n, i, j, k;
  ios::sync_with_stdio(false);
  ;
  cin >> m;
  for (i = 1; i <= m; i++) cin >> s[i];
  for (i = 1; i <= m; i++) {
    for (j = 0; j < s[i].length(); j++) {
      for (k = 1; k + j <= s[i].length(); k++) {
        int num = k, tmp = j;
        string b;
        while (num--) {
          b += s[i][tmp];
          tmp++;
        }
        a[b] = 1;
      }
    }
  }
  string ans;
  while (1) {
    for (i = 'a'; i <= 'z'; i++) {
      string ans1 = ans;
      ans1 += i;
      if (a[ans1] == 0) {
        cout << ans1 << endl;
        return 0;
      }
    }
    ans += 'z';
    if (ans.length() == 1) {
      ans = "a";
    } else {
      string tmp;
      for (i = 0; i < ans.length() - 1; i++) tmp[i] = ans[i];
      i = tmp.length() - 1;
      int flag = 0;
      while (flag == 0 && i >= 0) {
        if (tmp[i] == 'z')
          i--;
        else {
          tmp[i]++;
          flag = 1;
        }
      }
      if (flag == 0) {
        for (i = 0; i < ans.length(); i++) ans[i] = 'a';
      } else {
        ans = tmp;
      }
    }
  }
}
