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

class CodeforcesTask278BSolution:
    def __init__(self):
        self.result = ''
        self.n = 0
        self.names = []

    def read_input(self):
        self.n = int(input())
        for x in range(self.n):
            self.names.append(input())

    def process_task(self):
        used = {}
        for name in self.names:
            for x in range(len(name)):
                for y in range(x, len(name)):
                    sub = name[x:y + 1]
                    used[sub] = True
        cd = False
        for a in " abcdefghijklmnopqrstuvwxyz":
            for b in " abcdefghijklmnopqrstuvwxyz":
                for c in "abcdefghijklmnopqrstuvwxyz":
                    l1 = "" if a == " " else a
                    l2 = "" if b == " " else b
                    l3 = c
                    nn = l1 + l2 + l3
                    if nn not in used:
                        cd = True
                        break
                if cd:
                    break
            if cd:
                break

        self.result = nn

    def get_result(self):
        return self.result


if __name__ == "__main__":
    Solution = CodeforcesTask278BSolution()
    Solution.read_input()
    Solution.process_task()
    print(Solution.get_result())
