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;
import java.util.*;
public class Main {
    static String S[];
    static int n;
    static String letters = "abcdefghijklmnopqrstuvwxyz" ;
    static char pat[] ;
    static boolean found = false;
    public static boolean checkAll(String p){
        boolean b = false;
        for(int i =0;i<n;i++){
            b = b || (S[i].contains(p));
            if (b) return false;
        }
        return true;
    }
    public static String charToString(char cs[] ,int len){
        String temp = "" ;
        for(int i =0;i<len;i++){
            temp = temp + cs[i];
        }
        return temp;
    }
    public static void iteratePossition(int Possition,int MaxLength){
        if (found) return;
        if(Possition == MaxLength){
               String tmp = charToString(pat,MaxLength);
                        if (checkAll(tmp)) {
                            System.out.println(tmp);
                            found = true;
                        }
              return;
            
        }
        for(int i =0;i<26;i++){
            pat[Possition] = letters.charAt(i);
            iteratePossition(Possition+1,MaxLength);
        }
    }
    
    public static void find() {
        for(int currentLength = 1;currentLength < 22;currentLength++){
            iteratePossition(0,currentLength);
            if(found) return;
        }
    }
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        S = new String[n];
        pat = new char[33];
        for(int i =0;i<n;i++){
            S[i] = sc.next();
        }
        find();
    }
}
