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.*;
import java.io.*;
public class NewProblem
{
	public static void main(String[] args)
	{
		Scanner in = new Scanner(System.in);
		
		int n = in.nextInt();
		
		ArrayList<String> substrings = new ArrayList<String>();
		for(int x = 0; x < n; x++)
		{
			String name = in.next();
			
			for(int y = 0; y < name.length(); y++)
			{
				for(int z = y; z < name.length(); z++)
				{
					substrings.add(name.substring(y, z + 1));
				}
			}
		}
		
		Collections.sort(substrings);
		
		String result = "a";
		while(Collections.binarySearch(substrings, result) >= 0)
		{
			result = next(result);
		}
		
		System.out.println(result);
	}
	
	public static String next(String str)
	{
		StringBuilder ret = new StringBuilder();
		
		int index = str.length() - 1;
		while(str.charAt(index) == 'z')
		{
			ret.insert(0, 'a');
			index--;
			
			if(index < 0)
			{
				break;
			}
		}
		
		if(index >= 0)
		{
			ret.insert(0, (char)(str.charAt(index) + 1));
			ret.insert(0, str.substring(0, index));
		}
		else
		{
			ret.insert(0, 'a');
		}
		
		return ret.toString();
	}
}