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.io.InputStreamReader;
import java.math.BigInteger;
import java.util.Scanner;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author Trung Pham
 */
public class B {

    public static void main(String[] args) {
        Scanner in = new Scanner(new InputStreamReader(System.in));
        int N = in.nextInt();
        String[] data = new String[N];
        for (int i = 0; i < N; i++) {
            data[i] = in.next();
        }
        boolean found = false;
        for (int i = 0; i < 26 && !found; i++) {
            String a = "" + (char) (i + 'a');
         //   System.out.println(a);
            found = true;
            for (int j = 0; j < N; j++) {
              //  System.out.println(data[j]);
                if (data[j].contains(a)) {
                    found = false;
                    break;
                }
            }
            if (found) {
                System.out.println(a);
            }
        }
        if (!found) {

            for (int i = 0; i < 26 && !found; i++) {
                for (int j = 0; j < 26 && !found; j++) {
                    StringBuilder b = new StringBuilder();
                    b.append((char) (i + 'a')).append((char) (j + 'a'));
                   // System.out.println(i + " " + j);
                    found = true;
                    for (int k = 0; k < N; k++) {
                        if (data[k].contains(b.toString())) {
                            found = false;
                            break;
                        }
                    }
                    if (found) {
                        System.out.println(b.toString());
                    }
                }
            }
        }
    }
}
