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
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knojbur
pmfm
 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
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srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
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qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
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szahigtmklglrcocbo
znligfxkgxzkcfeu
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xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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itaubd
xcn
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x
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ci
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jos
qy
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ge
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nzoc
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wnjwjrf
t
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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 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String a[] = new String [705];
        int cnt = 0;
        for (int i = 97; i < 123; i++) {
            a[cnt] = (char)(i)+"";
            cnt++;
        }
        for (int i = 97; i < 123; i++) {
            for (int j = 97; j < 123; j++) {
                a[cnt] = (char)(i)+""+(char)(j)+"";
                cnt++;
            }
        }
//      System.out.println(cnt);
//      for (int i = 0; i < cnt; i++) {
//          System.out.println(a[i]);
//      }
        int n = sc.nextInt();
        String s = "";
        for (int i = 0; i <= n; i++) {
            String x= sc.nextLine();
            s += " "+ x;
        }
//      System.out.println(s);
        for (int i = 0; i < 702; i++) {
            if (s.indexOf(a[i])<0){
                System.out.println(a[i]);
                return;
            }
        }
    }

}