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.Scanner;

public class Main {

    public static void main(String[] args) {
    // write your code here
        Scanner scan = new Scanner(System.in);
        int strCount = scan.nextInt();
        StringBuilder[] words = new StringBuilder[35];
        scan.nextLine();
        for (int i=0;i<strCount;i++){
            words[i] = new StringBuilder(scan.nextLine());
        }
        StringBuilder myWord = new StringBuilder("z");
        boolean stop = false;
        int i = 97; //122
        int myWordCurChar = 0;
        int strLength = 0;

        while (!stop){
            myWord.setCharAt(myWordCurChar,(char)i);
            stop = true;
            for (int ii = 0;ii<strCount;ii++){
                if (words[ii].indexOf(myWord.toString())!=-1) {
                    stop = false;
                }
            }
            i++;
            if (i<=122 && !stop){
                StringBuilder tempWord = new StringBuilder(myWord);
                for (int curLetter=strLength;curLetter>myWordCurChar;curLetter--)
                    for (int curChar = 97;curChar<=122;curChar++){
                        tempWord.setCharAt(curLetter,(char)curChar);
                        for (int ii = 0;ii<strCount;ii++){
                            if (words[ii].indexOf(tempWord.toString())!=-1) {
                                stop = false;
                                myWord = tempWord;
                            }
                        }
                    }
            }
            if (i>122 && !stop) {
               // i = 97;
               // myWord.setCharAt(myWordCurChar,(char)i);
                boolean foundCharacter = false;
            //    if (myWordCurChar!=0)
                while(!foundCharacter){
                    if (myWord.charAt(myWordCurChar)=='z' && myWordCurChar==0){
                        myWord.setCharAt(myWordCurChar,'a');
                        strLength++;
                        myWordCurChar = strLength;
                        myWord.append('a');
                        foundCharacter = true;
                        i = 97;
                    } else {
                        myWordCurChar--;
                        if (myWordCurChar>=0 && myWord.charAt(myWordCurChar)!='z'){
                            foundCharacter = true;
                            i = (int)myWord.charAt(myWordCurChar);
                        }

                    }
                }

            }

        }
        System.out.print(myWord);

    }
}
