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.util.Arrays;
import java.util.Scanner;
import java.util.*;
public class Main {
    static String S[];
    static int n;
    static String letters = "abcdefghijklmnopqrstuvwxyz" ;
    public static boolean checkAll(String p){
        boolean b = false;
        for(int i =0;i<n;i++){
            b = b || (S[i].contains(p));
            if (b) return false;
        }
        return true;
    }
    
    public static void find() {
        char[] pat = new char[21];
        String tmp;
        for (int k = 1; k < 21; k++) {
            for (int i = 0; i < k; i++) {
                pat[i] = letters.charAt(0);
            }
            for (int i = k - 1; i > -1; i--) {
                for (int j = 0; j < 26; j++) {
                    pat[i] = letters.charAt(j);
                    tmp = "" ;
                    for(int r =0;r<k;r++){
                        tmp = tmp + pat[r];
                    }
                    if (checkAll(tmp)) {
                       System.out.println(tmp);
                      return;
                    }
                }
            }
        }
    }
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        S = new String[n];
        for(int i =0;i<n;i++){
            S[i] = sc.next();
        }
        find();
    }
}
