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.Scanner;


public class B170 {
	static String alph = "abcdefghijklmnopqrstuvwxyz";
	
	private static void genTest(){
		String a = new String("aa");
		int cx = 0, cy = 0;
		while (cy != 30){
			while (cx != 20){
				System.out.print(a);
				a = next(a);
				cx += 2;
			}
			System.out.println();
			cx = 0;
			cy += 1;
		}
	}

	private static String next(String str){
		char res[] = str.toCharArray();
		for (int i = res.length-1; i >= 0; i--){
			if (res[i] != 'z'){
				//char res[] = str.toCharArray();
				res[i] = (char) (str.charAt(i)+1);
				return new String(res);
			}
			else{
				res[i] = 'a';
			}
		}
		String a = new String(new char[str.length()+1]);
		a = a.replaceAll(".", "a");
		//System.out.println('"' + a +'"' + "!!!");
		return a;
	}
	
	public static void main(String[] args) {
		//genTest();
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		String[] strs= new String[n];
		in.nextLine();
		for (int i =0; i < n; i++){
			strs[i] = in.nextLine();
		}
		String res = new String("a");
		boolean found = false, bad = false;
		while(!found){
			bad = false;
			for (int i = 0; i < strs.length; i++){
				if (strs[i].contains(res)){
					//System.out.println(strs[i] + " contains " + res);
					res = next(res);
					bad = true;
					break;
				}
			}
			found = !bad;
		}
		System.out.print(res);
		in.close();
	}

}
