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.Arrays;
import java.util.Scanner;

public class r2 {

	static String[] str = new String[32];
	static String[] std = new String[27 * 27 * 27];

	static boolean[] has=new boolean[27*27*27];
	public static void main(String[] args) {
		Scanner cin = new Scanner(System.in);
		while (cin.hasNext()) {
			int n = cin.nextInt();
			for (int i = 0; i < n; i++) {
				str[i] = cin.next();				
			}
			Arrays.fill(has,false);
			for(int i=0;i<n;i++){
				for(int j=0;j<str[i].length();j++){
					int offset=str[i].charAt(j);
					offset-='a';
					has[offset]=true;
				}
			}
			boolean getans=false;
			for(int i=0;i<26;i++){
				if(has[i]==false){
					System.out.println((char)(i+'a'));
					getans=true;
					break;
				}
			}
			if(getans)
				continue;
			Arrays.fill(has,false);
			for(int i=0;i<n;i++){
				for(int j=0;j<str[i].length()-1;j++){
					int offset=(str[i].charAt(j)-'a')*26+(str[i].charAt(j+1)-'a');
					has[offset]=true;
				}
			}
			for(int i=0;i<26*26;i++){
				if(has[i]==false){
					System.out.println((char)((i/26)+'a')+""+(char)((i%26)+'a'));
					getans=true;
					break;
				}
			}
			if(getans)
				continue;
			Arrays.fill(has,false);
			for(int i=0;i<n;i++){
				for(int j=0;j<str[i].length()-2;j++){
					int offset=(str[i].charAt(j)-'a')*26*26+(str[i].charAt(j+1)-'a')*26+(str[i].charAt(j+2)-'a');
					has[offset]=true;
				}
			}
			for(int i=0;i<26*26*26;i++){
				if(has[i]==false){
					System.out.println((char)((i/26/26)+'a')+""+(char)(((i/26)%26)+'a')+""+(char)((i%26)+'a'));
					getans=true;
					break;
				}
			}
			
			

		}

	}

}
