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
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knojbur
pmfm
 2
hk
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 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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jrsrvsvbaiumlmkptn
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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
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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.Scanner;

public class B {

    static String[] name;
    static int n;

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        n = in.nextInt();
        name = new String[n];
        for (int i = 0; i < n; i++) {
            name[i] = in.next();
        }
        char ans = 'a';
        boolean ff = false;
        for (int j = 0; j < 26; j++) {
            ans = (char) ('a' + j);
            boolean f = false;
            for (int i = 0; i < n; i++) {

                if (name[i].indexOf(ans) >= 0) {
                    f = true;
                    break;
                }
            }
            if (!f) {
                ff = true;
                System.out.println(ans);
                break;
            }
        }
        if (ff) {

        } else {
            String jav = "";
            stop: for (char k = 'a'; k <= 'z'; k++) {
                jav = "" + k;
                for (int j = 0; j < 26; j++) {
                    boolean f = false;
                    for (int i = 0; i < n; i++) {
                        if (name[i].indexOf(jav + (char) ('a' + j)) >= 0) {
                            f = true;
                            break;
                        }
                    }
                    if (!f) {
                        System.out.println(jav + (char) ('a' + j));
                        break stop;
                    }
                }
            }

        }
    }

}
