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
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 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
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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
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qwad
gvz
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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;
import java.util.TreeSet;


public class ProblemA {

    
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        
        
        int n = s.nextInt();
        
        TreeSet<String> tree[] = new TreeSet[35];
        for (int i = 0; i < tree.length; i++) {
            tree[i] = new TreeSet<String>();
        }
        
        for (int i = 0; i < n; i++) {
            String str = s.next();
            int len = str.length();
            for (int j = 0; j < len; j++) {
                for (int j2 = j + 1; j2 <= len; j2++) {
                    String p = str.substring(j,j2);
//                  System.out.println(p);
                    tree[p.length()].add(p);
                }
            }
        }
        
        
        for(char c1 = 'a'; c1 <= 'z'; c1++){
            if(!tree[1].contains(c1+"")){
                System.out.println(c1);
                return ;
            }
        }
        
        for(char c1 = 'a'; c1 <= 'z'; c1++){
            for(char c2 = 'a'; c2 <= 'z'; c2++){
                String a = c1 + "" + c2;
                if(!tree[2].contains(a)){
                    System.out.println(a);
                    return;
                }
            }
        }
        for(char c1 = 'a'; c1 <= 'z'; c1++){
            for(char c2 = 'a'; c2 <= 'z'; c2++){
                for (char c3 = 'a'; c3 <= 'z'; c3++) {
                    String a = c1 + "" + c2+""+c3;
                    if(!tree[3].contains(a)){
                        System.out.println(a);
                        return;
                    }
                }
            }
        }
        
        for(char c1 = 'a'; c1 <= 'z'; c1++){
            for(char c2 = 'a'; c2 <= 'z'; c2++){
                for (char c3 = 'a'; c3 <= 'z'; c3++) {
                    for(char c4 = 'a'; c4 <= 'z'; c4++){
                        String a = c1 + "" + c2+""+c3+""+c4;
                        if(!tree[4].contains(a)){
                            System.out.println(a);
                            return;
                        }
                    }
                }
            }
        }
//      
//      System.out.println("End");
    }
    
    
}
