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

#codeforces #278B
import sys
from collections import deque

input_file=sys.stdin;
#input_file=open("input", "r");
strs = [];

def readline():
    return input_file.readline();

def hash_str(s):
    index = 0;
    ret = 0;
    for c in reversed(list(s)):
        ret += (ord(c) - ord('a') + 1)*(26**index);
        index += 1;
    return ret;

def reverse_hash_str(i):
    s = "";
    while i != 0:
        s += chr(i%26 + 96);
        i /= 26;
    return "".join(reversed(list(s)));

def str_range(l):
    st = "";
    en = "";
    for _ in xrange(l):
        st += "a";
        en += "z";


    return [hash_str(st), hash_str(en)];

if __name__ == '__main__':
    n = int(readline());
    for _ in xrange(n):
        strs.append(readline());

    for l in xrange(1, 27):
        ll = set()
        for s in strs:
            for st in xrange(len(s) - l):
                hs = hash_str(s[st:st + l]);
                ll.add(hs);

        r_bg, r_ed = str_range(l);
        if len(ll) < r_ed - r_bg + 1:
            #print "lack number in len ",l;
            #print sorted(ll);
            for i in range(r_bg, r_ed + 1):
                if i not in ll:
                    if i != 26 and i%26 == 0:
                        continue;
                    print reverse_hash_str(i);
                    exit(0);
