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.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class B {
    public static void main(String args[]) throws Exception{
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        String ss[]=new String[n];
        for(int i=0;i<n;i++){
            ss[i]=sc.next();
        }
        String s=""+'a';
        while(true){
            boolean isNot=false;
            for(int i=0;i<ss.length;i++){
                if(ss[i].indexOf(s)!=-1){
                    isNot=true;
                    break;
                }
            }
            if(!isNot){
               System.out.println(s);
               return;
            }
            s=""+next(s);



        }


    }

    public static String next(String s){

        char ch[]=new char[s.length()];
        boolean b=false;
        for(int i=s.length()-1;i>=0;i--){
            if(b){
                ch[i]=s.charAt(i);
                continue;
            }
            if(s.charAt(i)=='z'){
               ch[i]='z';

            }
            else{
                ch[i]+=(char)(s.charAt(i)+1);
                b=true;

            }
        }
        if(!b){
            StringBuffer sb=new StringBuffer();
            for(int i=0;i<s.length()+1;i++){
                sb.append('a');

            }
            return sb.toString();
        }
        StringBuffer sb=new StringBuffer(30);
        for(int i=0;i<ch.length;i++){
            sb.append(ch[i]);
        }
        return sb.toString();
    }
}
