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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class ProblemCode {
    static int n;
    static String [] prev;
    static boolean chars [] = new boolean[26]; 
    static String total = "";
    
    public void input() throws IOException {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        n = Integer.parseInt(br.readLine());
        prev = new String[n];
        
        for(int i=0;i<n;i++){
            prev[i] = br.readLine();
        }
    }
    public char letterCheck(){
        for(int i=0;i<n;i++){
            for(int j=0;j<prev[i].length();j++){
                            char b = (prev[i].charAt(j));
                            int index;
                        index = b-'a';
                chars[index]=true;
            }
        }
        
        for(int i = 0; i<26;i++){
            if(!chars[i]) {
                        return (char)('a'+i);
                    }  
        }
        return ' ';
    }
        public void algo(){
            
            for(int i=0;i<n;i++){
                total += prev[i] + "|";
            }
            char out = letterCheck();
            if(out != ' ')
                System.out.println(out);
            else {
            String cur = "aa";
            while(total.indexOf(cur)!=-1){
                cur = next(cur);
            }
            System.out.println(cur);
            }
        }
        public String next(String cur){
            char lastChar = cur.charAt(cur.length()-1);
            String base =  cur.substring(0, cur.length()-1);

            if(lastChar!='z'){
                String last;
                last = (char)(lastChar + 1) + "";
                cur = base + last;
            }
            else if(base.length()!=0){
                base = next(base);
                String last = "a";
                cur = base + last;
            }
            else{
                cur = "aa";
            }
            return cur;
        }
    public static void main(String[] args) throws IOException {
        ProblemCode pc = new ProblemCode();
        pc.input();
        pc.algo();
        
    }

            

}