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
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
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svmytmcfhc
jrsrvsvbaiumlmkptn
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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
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q
cvdu
a
jos
qy
fjf
nayke
ge
s
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qwad
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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 problemB {
    
    
    
    static public void main(String[] arg){
        
        
        
        Scanner in = new Scanner(System.in);
        
        int n=Integer.parseInt(in.nextLine());
        String buf[]=new String [n];
        
        boolean ok[] = new boolean [50];
        for(int i=0;i<n;i++)
            buf[i]=in.nextLine();
        
        
        ok=new boolean[50];
        for(int i=0; i<n;i++)
        for(int j=0; j<buf[i].length();j++)
                    ok[(int)(buf[i].charAt(j) - 'a')]=true;
                    
    

    for(int i=0; i<26; i++)
    {
        if (!ok[i]) 
        {
            
            System.out.println((char)(i+'a'));
            return;
            
        }
    }
        
        int k=1;
        
        
        
        for(;;){
            
            
            ok=new boolean[50];
            for(int i=0; i<n;i++)
            { 
                for(int j=0; j<buf[i].length();j++)
                { int l=0;
                    while(j<buf[i].length()-1 && buf[i].charAt(j)=='a')
                    {
                        l++;j++;
                        if (k-l==0)
                        {// System.out.println(buf[i].charAt(j));
                            ok[(int)(buf[i].charAt(j) - 'a')]=true;
                            if (buf[i].charAt(j)=='a') l--;
                        }
                    }
                }
            }
            
                        for(int i=0; i<26; i++)
            {
                if (!ok[i]) 
                {
                    for(int j=0; j<k;j++)
                        System.out.print('a');
                    System.out.println((char)(i+'a'));
                    return;
                    
                }
            }
            k++;
        }
        
        
    }
}
