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 New_Problem {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num = input.nextInt();
		String s = "";
		for (int i = 0; i < num; i++) {
			s += input.next();
			s+="*";
		}
		String title = "";
		char[] arr = new char[26];
		arr[0] = 'a';
		arr[1] = 'b';
		arr[2] = 'c';
		arr[3] = 'd';
		arr[4] = 'e';
		arr[5] = 'f';
		arr[6] = 'g';
		arr[7] = 'h';
		arr[8] = 'i';
		arr[9] = 'j';
		arr[10] = 'k';
		arr[11] = 'l';
		arr[12] = 'm';
		arr[13] = 'n';
		arr[14] = 'o';
		arr[15] = 'p';
		arr[16] = 'q';
		arr[17] = 'r';
		arr[18] = 's';
		arr[19] = 't';
		arr[20] = 'u';
		arr[21] = 'v';
		arr[22] = 'w';
		arr[23] = 'x';
		arr[24] = 'y';
		arr[25] = 'z';
		int k=0;
		char c ;
		while (true) {
			for (int i =0 ; i < 26; i++) {
				c=arr[i];
				if(!s.contains(title+c)){
					System.out.println(title+c);
					return;
				}
				
			}
			if(title.length()!=26){
			title+=arr[k];
			}else{
				title=arr[++k]+"";
			}
			
		}

	}
}
