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

//package Div_170;
//Xudo xoxlasa Accepted

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.TreeMap;
import java.util.TreeSet;

public class B {
public static void main(String[] args) {
	Scanner sc=new Scanner(System.in);
	int n=sc.nextInt();
	String [] s=new String[n+1];
	TreeSet<String>st=new TreeSet<String>();
	Map<Integer, TreeSet<String>>mp=new HashMap<Integer, TreeSet<String>>();
	String  hamma="";
	for (int i =1; i <=20; i++) {
		mp.put(i, st);
	}
	for (int i =1; i <=n; i++) {
		s[i]=sc.next();
		hamma+=s[i]+"+";
		for (int j =1; j <=s[i].length(); j++) {
			for (int j2 =0; j2 <=s[i].length()-j; j2++) {
				st=new TreeSet(mp.get(j));
				st.add(s[i].substring(j2,j2+j));
				mp.put(j, st);
			}
		}
	}
	for(Integer k:mp.keySet()){
		if(!mp.get(k).isEmpty()){
		st=new TreeSet(mp.get(k));
	String ss=st.first();
	ss=yangila(ss);
	int g=0;
	String ggg="";
	for (String i:st) {
		ggg=i;
		if(g==0){g++;continue;}
		if(i.equals(ss)){
			ss=yangila(ss);
		}else{
			System.out.println(ss);
			return;
		}
	}

}else{
	for (int i =1; i <=k; i++) {
		System.out.print("a");
	}
	return;
}
	}
		
	}

private static String yangila(String ss) {
	char g='a';
	int k=ss.length();
		for (int i =k-1; i>=0; i--) {
			if(ss.charAt(i)!='z'){
				g=ss.charAt(i);
				g++;
				ss=ss.substring(0,i)+(g)+ss.substring(i+1,ss.length());
				return ss;
			}else{
			ss=ss.substring(0,i)+(g)+ss.substring(i+1,ss.length());
		}}
		
return ss;		
}
}
