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
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.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class C {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);

        int n = Integer.parseInt(s.nextLine());
        String[] arr = new String[n];

        for (int i = 0; i < arr.length; i++)
            arr[i] = s.nextLine();

        int[] alpha = new int[26];

        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[i].length(); j++)
                alpha[arr[i].charAt(j) - 'a']++;
        }

        int ind = -1;

        for (int i = 0; i < alpha.length; i++) {
            if (alpha[i] == 0) {
                ind = i;
                break;
            }
        }

        if (ind != -1) {
            System.out.println((char) (ind + 'a'));
        } else {

            String res = "aa";

            
             al : while (true) {
                boolean flg = false;
                
                for (int i = 0; i < 26; i++) {
                    
                    for (int k = 0; k < arr.length; k++)
                        if (arr[k].contains(res)) {
                            flg = true;
                            break ;
                        }
                    
                    if(flg){
                        res = res.substring(0,res.length()-1) + (char) (i+'a');
                        flg = false;
                    }
                        else 
                            break al;
                    
                    
                }
                if(res.charAt(res.length()-1) == 'z'){
                    res = res.replace('z','a');
                    
                    res += "a";
                }
            }
            
            System.out.println(res);
        }
    }
}