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
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sshgcxsky
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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
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qwad
gvz
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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
{
    static Set<String> subs;
    public static void main(String [] args)
    {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();

        subs = new TreeSet<String>();

        scanner.nextLine();
        for(int c = 0; c < n; c++)
        {
            String str = scanner.nextLine();
            for (int i = 0; i < str.length(); i++)
                for (int j = i+1; j <= str.length(); j++)
                    subs.add(str.substring(i, j));
        }

        String res = "a";

        while (subs.contains(res))
            res = next(new StringBuilder(res), res.length()-1);

        System.out.println(res);



    }

    private static String next(StringBuilder str, int n)
    {
        int at = n;

        str = str.replace(n, n+1, (char)(str.charAt(n)+1)+"");
        while (str.charAt(at) > 'z')
        {
            str = str.replace(at, at+1, "a");
            at--;
            if(at < 0)
                return "a"+str;
            str = str.replace(at, at+1, (char)(str.charAt(at)+1)+"");
        }

        return str.toString();

    }

}
