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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svmytmcfhc
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
zbi
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;
char buf[99][99];
int n;
void Deal() {
  int mark;
  int len = 1;
  char res[99];
  memset(res, 0, sizeof(res));
  res[0] = 'a';
  while (1) {
    int i;
    mark = 0;
    for (i = 1; i <= n; i++) {
      if (strstr(buf[i], res) == 0) {
        mark += 1;
      }
    }
    if (mark == n) {
      printf("%s\n", res);
      return;
    } else {
      res[len - 1] += 1;
      for (int j = len - 1; j >= 0; j--) {
        if (res[j] > 'z') {
          if (j == 0) {
            for (; j <= len; j++) res[j] = 'a';
            len += 1;
          } else {
            res[j - 1] += 1;
          }
        }
      }
    }
  }
}
int main() {
  while (scanf("%d", &n) != EOF) {
    memset(buf, 0, sizeof(buf));
    for (int i = 1; i <= n; i++) {
      scanf("%s", &buf[i]);
    }
    Deal();
  }
  return 0;
}
