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.*;
import java.io.*;
import java.nio.CharBuffer;

public class ProblemB {
	Scanner sc = new Scanner(System.in);
	PrintStream out = System.out;

	String[] titles;
	StringBuilder nt = new StringBuilder(20); 
	
	boolean isSubString() {
	   for (String title : titles) {
		  if (title.contains(nt)) {
			  return true;
		  }
	   }
	   return false;
	}

	boolean tryseq(int n) {
		int length = nt.length();
		if (n == 0) {
			//out.println(nt);
			return !isSubString();
		} 
		
		for (char x = 'a'; x <= 'z'; x++) {
			nt.setCharAt(length-n, x);
			if (tryseq(n-1)) {
				return true;
			}
		}
		return false;
	
	}
	
	void start() {
		
		int n = sc.nextInt();
		titles = new String[n];
		for (int i = 0; i < n; i++) {
			titles[i] = sc.next();
		}
		
		for (int i = 1; i <= 20; i++) {
			nt.append('a');
			if (tryseq(i)) {
				out.print(nt);
				return;
			}
		}
		
	}
	
	public static void main(String[] args) {
		new ProblemB().start();
	}

}
