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
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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
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.*;

public class NewProblem {
    
    public static void main( String[] args ) throws IOException {
        
        BufferedReader in = new BufferedReader( new InputStreamReader( System.in ) );
        PrintWriter out = new PrintWriter( new BufferedWriter( new OutputStreamWriter( System.out ) ) );
        
        String[] letters = { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };
        
        int n = Integer.parseInt( in.readLine() );
        
        String[] names = new String[ n ];
        
        for( int i = 0; i < names.length; i++ ) {
            
            names[ i ] = in.readLine();
            
        }
        
        for( int i = 0; i < letters.length; i++ ) {
            
            boolean present = false;
            for( int j = 0; j < names.length; j++ ) {
                
                if( names[ j ].contains( letters[ i ] ) ) {
                    present = true; 
                    break;
                }
            }
            
            if( !present ) {
                out.println( letters[ i ] );
                out.flush();
                out.close();
                System.exit( 0 );
            }
        }
        
        ArrayList<String> subStringsList = new ArrayList<String>();
        for( int i = 0; i < letters.length; i++ ) {
            
            for( int j = 0; j < letters.length; j++ ) {
                
                String subString = letters[ i ] + letters[ j ];
                boolean present = false;
                for( int k = 0; k < names.length; k++ ) {
                    
                    if( names[ k ].contains( subString ) ) {
                        present = true;
                        break;
                    }
                }
                
                if( !present )
                    subStringsList.add( subString );
            }
        }
        
        String[] subStrings = new String[ subStringsList.size() ];
        
        for( int i = 0; i < subStrings.length; i++ )
            subStrings[ i ] = subStringsList.get( i );
        
        Arrays.sort( subStrings );
        
        out.println( subStrings[ 0 ] );
        out.flush();
        out.close();
        
    }
}