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.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author user
 */
public class CFround170div2B {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        //System.out.println("abc".contains("bc"));
        int t = Integer.parseInt(br.readLine());

        String[] in = new String[t];

        for (int a = 0; a < t; a++) {
            in[a] = br.readLine();
        }

        String[] abc = new String[26];
        String klm = "abcdefghijklmnopqrstuvwxyz";
        for (int a = 0; a < 26; a++) {
            abc[a] = "" + klm.charAt(a);
        }
       // int c = 0;
        boolean ada = true;
        
            //c++;
            String s = ""+klm.charAt(0);

            for (int b = 0; b < 26; b++) {
               // System.out.println(""+klm.charAt(b));
                
                    ada = true;
                    int c = 0;
                    for (int a = 0; a < t; a++) {
                        if (!in[a].contains(""+klm.charAt(b))) {
                            c++;
                            //System.out.println(""+ klm.charAt(b) + c);
                            
                        }
                    }
                    //System.out.println(klm.charAt(b));
                    if (c==t) {
                        System.out.println(""+klm.charAt(b));
                        System.exit(0);
                    }
                
            }
            
            for (int b = 0; b < 26; b++) {

                for(int d = 0; d < 26; d++){
                    //System.out.println(""+klm.charAt(b)+klm.charAt(d));
                    int c = 0;
                    ada = true;
                    for (int a = 0; a < t; a++) {
                        if (!in[a].contains(""+klm.charAt(b)+klm.charAt(d))) {
                            c++; 
                        }
                    }
                    if (c==t) {
                        System.out.println(""+klm.charAt(b)+klm.charAt(d));
                        System.exit(0);
                    }
                }
                
            }
        

    }
}
