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.io.*;
import java.util.*;
import java.lang.Math;

public class R170Div2B {
	static String[] a = new String[40];
	
	public static int n =0;
	public static char next1(char c){
		int ascii = (int)c;
		if (ascii>122){
			return '0';
		}else{
			ascii++;
			return (char) ascii;
		}
	}
	
	//String abc = scanner.next();
	//int def = (int)abc.charAt(0);
	//System.out.println(def);
	
	public static boolean orig(String ha){
		for (int i=0; i<n; i++){
			if (a[i].contains(ha)){
				return false;
			}
		}
		return true;
	}
	
	public static String single(){
	    String t="a";
		while (t.compareTo("z")<=0){
			if (orig(t)){
				return t;
			}
			t = "" + next1(t.charAt(0));
		}
		return "0";
	}
	
	public static String dual(){
		String t="aa";
		while (t.compareTo("zz")<=0){
			if (orig(t)){
				return t;
			}
			
			if (t.charAt(1)=='z'){
				char t0 = next1(t.charAt(0));
				t = t0 + "a";
			}else if (t.charAt(0) == '0'){
				return "00";
			}else{
				t = "" + t.charAt(0) + next1(t.charAt(1));
			}
		}
		return "00";
	}
	
	public static String trio(String t){
		while (t.compareTo("zzz")<=0){
			if (orig(t)){
				return t;
			}
			
			if (t.charAt(2)=='z'){
				char t1 = next1(t.charAt(1));
				t = t.charAt(0) + t1 + "a";
			}else if (t.charAt(1) == '0'){
				return trio(next1(t.charAt(0))+"aa");
			}else{
				t = "" + t.charAt(0) + t.charAt(1) + next1(t.charAt(2));
			}
		}
		
		return "000";
	}
	
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		
		n= Integer.parseInt(scanner.next());
	
		for (int i=0; i<n; i++){
			a[i]= scanner.next();
		}
		
		String ans = single();
		if (ans != "0"){
			System.out.println(ans);
		}else{
			String ans2 = dual();
			if (ans2 != "00"){
				System.out.println(ans2);
			}else{
				String ans3 = trio("aaa");
				if (ans3 != "000"){
					System.out.println(ans3);
				}else{
					System.out.println("aaaa");
				}
			}
		}
	}
}
