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

//NewProblem

import java.util.*;

public class NewProblem{
  private static ArrayList<String> titles;
  private static final String alphabets = "abcdefghijklmnopqrstuvwxyz";
  private static final String base = "aaaaaaaaaaaaaaaaaaaa";
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    String target = "a";
    boolean found = false;
    titles = new ArrayList<String>();
    for(int i=0;i<N;i++){
      titles.add(sc.next());
    }
    while(true){
      found = false;
      for(int i=0;i<titles.size()&&!found;i++){
        if(titles.get(i).contains(target)){
          found = true; 
          target = upgrade(target);
        }
      }
      if(!found)
        break;
    }
    System.out.println(target);
  }
  private static String upgrade(String s){
    int sLen = s.length();
    int newCharIndex = s.charAt(sLen-1)-97+1;
    String newChar = "";
    if(s.substring(sLen-1).equals("z")){
      s = base.substring(0,sLen+1);
    }
    else if(sLen==1){
      newChar = alphabets.substring(newCharIndex,newCharIndex+1);
      s = newChar;
      
    }
    else{
      newChar = alphabets.substring(newCharIndex,newCharIndex+1);
      s = s.substring(0,sLen-1)+newChar;
    }
    //System.out.println(s+" "+newChar);
    return s;
    
  }

}