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;
char name[31][21], c[21];
int n, f[21];
bool flag;
bool kmp(char a[], char b[]) {
  int n, m, i, j;
  n = strlen(a);
  m = strlen(b);
  f[0] = j = -1;
  for (i = 1; i < m; i++) {
    while (j > -1 && b[j + 1] != b[i]) j = f[j];
    if (b[j + 1] == b[i]) j++;
    f[i] = j;
  }
  j = -1;
  for (i = 0; i < n; i++) {
    while (j > -1 && b[j + 1] != a[i]) j = f[j];
    if (b[j + 1] == a[i]) j++;
    if (j == m - 1) return true;
  }
  return false;
}
void dfs(int step, int t) {
  if (flag == true) return;
  if (t < step) {
    char i;
    for (i = 'a'; i <= 'z' && flag == false; i++) {
      c[t] = i;
      dfs(step, t + 1);
    }
  } else {
    int j;
    for (j = 1; j <= n; j++)
      if (kmp(name[j], c)) return;
    flag = true;
  }
}
int main() {
  int i, k = 0;
  scanf("%d", &n);
  for (i = 1; i <= n; i++) scanf("%s", name[i]);
  flag = false;
  while (flag == false) {
    k++;
    dfs(k, 0);
  }
  printf("%s\n", c);
  return 0;
}
