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
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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.lang.reflect.Array;
import java.util.Arrays;
import java.util.Scanner;


public class name {
	
	String[] perm(String[] alph,int n)
	{
		if(n==0)
		{
			String[] ret = {};
			return ret;
		}
		if(n==1)return alph;
		String[] ret = new String[(int)Math.pow(26,n)];
		String[] t = perm(alph,n-1);
		for(int i=0;i<26;i++)
		{
			for(int j=0;j<t.length;j++)
			{
				ret[(t.length)*i+j] = alph[i]+t[j];
			}
		}
		return ret;
	}
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		 String[] alph = new String[26];
		int j=0;
		// TODO Auto-generated method stub
		for(char c='a';c<='z';c++)
		{
			alph[j++]=""+c;
		}
		Scanner sc = new Scanner(System.in);
		int number = sc.nextInt();
		String[] arr = new String[number];
		//System.out.println(number);
		sc.nextLine();
		for(int i=0;i<number;i++)
		{
			arr[i] = sc.nextLine();
		}
	//System.out.println(arr.toString());
		int k=1;
		int flag =0;
		name obj = new name();
		String ans = "";
		while(flag==0)
		{
			//System.out.println(k);
			String[] to_check = obj.perm(alph,k++);
			Arrays.sort(to_check);
			
			int l=0;
			for(l=0;l<to_check.length;l++)
			{
				if(flag==1)break;
				int p=0;
				for( p=0;p<number;p++)
				{
					if(arr[p].contains(to_check[l]))break;
					
				}		
				if(p==number)
					{
						flag=1;
						ans = to_check[l];
					}
				
			}
		}
		System.out.println(ans);
		
	}

}
