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;
struct str {
  char s[30];
};
int n, t, pos, cnt = 0;
char a[30];
str b[200000];
bool cmp(str aa, str bb) {
  if (strlen(aa.s) < strlen(bb.s))
    return true;
  else if (strlen(aa.s) > strlen(bb.s))
    return false;
  else if (strcmp(aa.s, bb.s) <= 0)
    return true;
  else
    return false;
}
int main() {
  memset(b, 0, sizeof(b));
  memset(a, 0, sizeof(a));
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    scanf("%s", a);
    for (int j = 1; j <= min((int)strlen(a), 2); ++j) {
      for (int k = 0; k < (int)strlen(a) - j + 1; ++k) {
        for (int p = 0; p < j; ++p) {
          b[cnt].s[p] = a[k + p];
        }
        b[cnt].s[j] = 0;
        cnt++;
      }
    }
  }
  sort(&b[0], &b[cnt], cmp);
  if (strcmp(b[0].s, "a") != 0)
    printf("a");
  else
    for (int i = 0; i < cnt; ++i) {
      if (strcmp(b[i].s, b[i + 1].s) == 0) continue;
      pos = strlen(b[i].s) - 1;
      while (pos >= 0 && b[i].s[pos] == 'z') {
        b[i].s[pos] = 'a';
        pos--;
      }
      if (pos < 0) {
        for (int j = strlen(b[i].s); j > 0; --j) {
          b[i].s[j] = b[i].s[j - 1];
        }
        b[i].s[0] = 'a';
      } else
        b[i].s[pos] = b[i].s[pos] + 1;
      if (strcmp(b[i].s, b[i + 1].s) != 0) {
        printf("%s\n", b[i].s);
        break;
      }
    }
  return 0;
}
