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.Arrays;
import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * User: Mohammad Ali Tayefi
 * Date: 2/28/13
 * Time: 7:23 PM
 * To change this template use File | Settings | File Templates.
 */
public class NewProblem {
    public static void main(String[] args){
        Scanner inp=new Scanner(System.in);
        int n=inp.nextInt();
        String[] used=new String[n];
        for(int i=0;i<n;i++)used[i]=inp.next();
        char[] alphabet=new char[26];
        for(int i=0;i<n;i++){
            for(int j=0;j<used[i].length();j++)alphabet[used[i].charAt(j)-97]=used[i].charAt(j);
        }
        for(int i=0;i<26;i++){
            if(alphabet[i]!=i+97){
                char a=(char)(i+97);
                System.out.println(a);
                System.exit(0);
            }
        }
        int minString=0;
        String original="";
        boolean finished=false;
        boolean[] toBeChecked=new boolean[n];
        for(int i=0;i<n;i++)toBeChecked[i]=true;
        int charAt=0;
        int minLength=100;
        char maxAsciiUsedInLast=(char)96;
        while(!finished){
            char minChar=(char)123;
            for(int i=0;i<n;i++){
                if(toBeChecked[i]){
                    if(used[i].charAt(charAt)<minChar){
                        minChar=used[i].charAt(charAt);
                    }
                }
            }
            original=original+String.valueOf(minChar);
            for(int i=0;i<n;i++){
                if(toBeChecked[i]){
                    if(used[i].charAt(charAt)!=minChar)toBeChecked[i]=false;
                }
                if(toBeChecked[i]){
                    if(used[i].length()<minLength)minLength = used[i].length();
                }
            }
            //System.out.println(Arrays.toString(toBeChecked));
            if(charAt+2 ==minLength){
                for(int i=0;i<n;i++){
                    if(toBeChecked[i]){
                        if(used[i].charAt(charAt+1)>maxAsciiUsedInLast)maxAsciiUsedInLast=used[i].charAt(charAt+1);
                    }
                }
                if(maxAsciiUsedInLast!='z')original=original+String.valueOf((char)(maxAsciiUsedInLast+1));
                else {
                    original=original+String.valueOf("za");
                }
                finished = true;
            }
            System.out.println(original);
            finished=true;
            charAt++;
        }

    }
}
