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;
const int MAXN = 26 + 1;
struct Trie {
  Trie* next[MAXN];
} root;
void CreateTrie(char* s) {
  int len = strlen(s), i, j, id;
  Trie* t = &root;
  for (i = 0; i < len; i++) {
    id = s[i] - 'a';
    if (t->next[id] == NULL) {
      t->next[id] = new Trie;
      for (j = 0; j < MAXN; j++) t->next[id]->next[j] = NULL;
      t = t->next[id];
    } else {
      t = t->next[id];
    }
  }
}
int FindTrie(char* s) {
  int len = strlen(s), i, id;
  Trie* t = &root;
  for (i = 0; i < len; i++) {
    id = s[i] - 'a';
    if (t->next[id] == NULL)
      return 0;
    else
      t = t->next[id];
  }
  return 1;
}
void DelTrie(Trie* t) {
  int i;
  for (i = 0; i < MAXN; i++) {
    if (t->next[i] != NULL) DelTrie(t->next[i]);
  }
  delete t;
}
int main() {
  bool flag;
  char s[30], ts[30];
  int n, i, j, len, h, k;
  for (i = 0; i < MAXN; i++) root.next[i] = NULL;
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    scanf("%s", s);
    len = strlen(s);
    for (j = 0; j < len; j++) {
      CreateTrie(s + j);
    }
  }
  for (i = 1; i <= 20; i++) {
    for (j = 0; j < i; j++) {
      s[j] = 'a';
    }
    s[j] = '\0';
    if (FindTrie(s) == 0) {
      printf("%s\n", s);
      return 0;
    }
    flag = true;
    while (flag) {
      j = i - 1;
      bool mark = 0;
      while (s[j] == 'z') {
        if (j - 1 < 0) {
          flag = false;
          break;
        }
        s[j - 1]++;
        s[j] = 'a';
        mark = 1;
        j--;
      }
      if (mark == 0) s[i - 1]++;
      if (FindTrie(s) == 0) {
        printf("%s\n", s);
        return 0;
      }
    }
  }
  DelTrie(&root);
  return 0;
}
