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.*;
public class Main
{
    public static void main(String [] args)
    {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        ArrayList<String> titles = new ArrayList<String>();
        SortedSet<Character> ch = new TreeSet<Character>();

        scan.nextLine();
        for(int i = 0; i < n; i++)
        {
            String item = scan.nextLine().trim();
            if(titles.contains(item))
                continue;
            titles.add(item);
            for(int j = 0; j < item.length(); j++)
                ch.add(item.charAt(j));
        }


        if(ch.size() < 26)
        {
            char j = 'a';
            for(Character i : ch)
                if(i == j)
                    j++;
                else
                {
                    System.out.println(j);
                    break;
                }
        }

        else
        {
            Collections.sort(titles, new Comparator<String>() {
                @Override
                public int compare(String o1, String o2)
                {
                    return o1.length() - o2.length();
                }
            });
            char lex = 'z';
            for(int i = 0; i < titles.size() ; i++)
                if(titles.get(i).length() == 2)
                    lex = (char)Math.min(lex, titles.get(i).charAt(1));

            System.out.println("a"+(char)(lex+1));
        }

    }
}
