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.IOException;
//You can include standard Java language libraries that you want use.//

import java.util.Scanner;

public class A{

/*	    public static StreamTokenizer in;
	    public static PrintStream out;

	    public static String readString() throws IOException {
	        in.nextToken();
	        return  in.sval;
	        }

	    public static double readDouble() throws IOException {
	        in.nextToken();
	        return in.nval;
	        }

	    public static int readInt() throws IOException {
	        in.nextToken();
	        return (int) in.nval;
	    }*/

	
     public void solution(String inputFilePath, String usrOutputFilePath ) throws IOException{    	 
//    	 in = new StreamTokenizer(new FileReader(inputFilePath) );

         Scanner rd = new Scanner(System.in);
         
//        	 int n = readInt();
         	int n = rd.nextInt();
         	String alp[] = {"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"};

        	 String d[] = new String[n];

        	 
        	 for (int i=0; i<n; ++i)
        	 {
        		 d[i] = rd.next();
        	 }

        	 for (int i=0; i<100000; ++i)
        	 {
        		 int word[] = new int[30];
        		 int k =i;
        		 int j=0;
        		 while (k>0)
        		 {
        			 word[j] = k%26;
        			 j++;
        			 k /=26;
        		 }
        		 String ans = "";
        		 for (int j2 = j-1; j2 >=0; j2--) {
					ans = alp[word[j2]] + ans;
				}
boolean tt=false;
            	 for (int j3=0; j3<n; ++j3)
            	 {
            		 if (d[j3].indexOf(ans)!=-1)
            			 tt=true;
            	 }
            	 if (!tt)
            	 {	 
            		 System.out.println(ans);
            		 return;
            	 }
        	 }
        	 

        	 
         }

 	public static void main(String[] args) throws IOException {
		A test = new A();
		test.solution("in.txt", "out.txt");

	}
     
}



