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

import java.util.Scanner;

public class NewProblem
{
	public static void Next(StringBuilder sb)
	{
		int idx = sb.length() - 1;
		while (idx >= 0)
		{
			if (sb.charAt(idx) == 'z')
			{
				--idx;
			}
			else
			{
				sb.setCharAt(idx, (char)(sb.charAt(idx) + 1));
				return;
			}
		}
		int len = sb.length() + 1;
		sb.delete(0, sb.length());
		for (int i = 0; i < len; ++i)
		{
			sb.append('a');
		}
	}

	public static void main(String[] args)
	{
		Scanner input = new Scanner(System.in);
		int n = input.nextInt();
		input.nextLine();
		String[] s = new String[n];
		for (int i = 0; i < n; ++i)
		{
			s[i] = input.nextLine();
		}
		StringBuilder sb = new StringBuilder();
		boolean flag = false;
		while (!flag)
		{
			Next(sb);
			flag = true;
			for (int i = 0; i < n; ++i)
			{
				if (s[i].indexOf(sb.toString()) != -1)
				{
					flag = false;
					break;
				}
			}
		}
		System.out.println(sb);
	}
}