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.*;

public class NewProblem {
  
  static String[] names;
  
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    
    int noOfTitles = sc.nextInt();
    names = new String[noOfTitles];
    
    sc.nextLine();
    
    for(int i = 0; i < noOfTitles; i++) {
      names[i] = sc.nextLine();
    }
    
    String answer = "a";
    
    //System.out.println((char)('a'+26));
    
    while(hasSubstring(answer)) {
      answer = incrementString(answer);
    }
    
    System.out.println(answer);
    
    sc.close();
  }
  
  public static boolean hasSubstring(String input) {
    for(int i = 0; i < names.length; i++) {
      if(names[i].contains(input)) {
        return true;
      }
    }
    return false;
  }
  
  public static String incrementString(String input) {
    int currentPos = input.length() - 1;
    StringBuilder genWord = new StringBuilder(input);
    
    while(currentPos >= 0 && currentPos < input.length()) {
      char currentChar = genWord.charAt(currentPos);
      currentChar++;
      
      if(currentChar > ('a' + 25)) { 
        genWord.setCharAt(currentPos, 'a');
        currentPos--;
      }else{
        genWord.setCharAt(currentPos, currentChar);
        currentPos++;
      }
    }
    
    input = genWord.toString();
    
    if(currentPos < 0) {
      input = "a" + input;
    }
    
    return input;
    
  }
  
  
}