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
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 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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sshgcxsky
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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;
const double ep = 1e-9;
const long long M = 1000010;
const long long IN = 1e15;
char str[35][25];
long long pr[M];
int main() {
  long long n, a, b, k;
  while (scanf("%lld", &n) != EOF) {
    for (long long i = 0; i < n; i++) {
      scanf("%s", str[i]);
    }
    bool flag = false, fflag = false;
    char sstr[5];
    for (char j = 'a'; j <= 'z' && !fflag; j++) {
      flag = true;
      sstr[0] = j;
      sstr[1] = '\0';
      for (long long i = 0; i < n; i++) {
        if (strstr(str[i], sstr) != 0) {
          flag = false;
        }
      }
      if (flag) {
        printf("%s\n", sstr);
        fflag = true;
      }
    }
    for (char k = 'a'; k <= 'z' && !fflag; k++) {
      for (char j = 'a'; j <= 'z' && !fflag; j++) {
        flag = true;
        sstr[0] = k;
        sstr[1] = j;
        sstr[2] = '\0';
        for (long long i = 0; i < n; i++) {
          if (strstr(str[i], sstr) != 0) {
            flag = false;
          }
        }
        if (flag) {
          printf("%s\n", sstr);
          fflag = true;
        }
      }
    }
  }
  return 0;
}
