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
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 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
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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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Set;
import java.util.StringTokenizer;
import java.util.TreeSet;
public class B
{
    public static BufferedReader br;
    public static StringTokenizer inp;
    public static PrintWriter out;
    
    
    public static Set<String> set;
    public static void main(String[] args)  throws IOException
    {
        br = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);
        read();
        int n = nextInt();
        set = new TreeSet<String>();
        for(int i = 0 ; i < n ; i++)
        {
            String line = br.readLine();
            for(int k = 0 ; k < line.length() ; k++)
            {
                for(int j = k+1 ; j <= line.length() ; j++)
                {
                    set.add(line.substring(k,j));
                }
            }
        }
        int m = set.size();
        String s[]= new String[1];
        s=set.toArray(new String[0]);
        Arrays.sort(s, new Comparator<String>() {

            public int compare(String o1, String o2) {
                if(o1.length()<o2.length())return -1;
                else if(o1.length()>o2.length())return 1;
                return o1.compareTo(o2);
                
            }
        });
        String ans = "";
        if(s[0].equals("a"))
        {
            for(int i = 0 ; i < m-1 ; i++)
            {
                String t = nex(s[i]);
                if(!t.equals(s[i+1]))
                {
                    ans = t;
                    break;
                }
            }
        }
        else
        {
            ans = "a";
        }
        if(ans.equals(""))
            ans = nex(s[s.length-1]);
        out.println(ans);
        
        br.close();
        out.close();
    }
    
    public static String nex(String a)
    {
        char c[] = (('a')+a).toCharArray();
        for(int i = c.length-1 ; i>0 ; i--)
        {
            if(c[i]<'z')
            {
                c[i]++;
                return String.valueOf(c).substring(1);
            }
            else
            {
                c[i]='a';
            }
        }
        return String.valueOf(c);
    }
    
    
    public static void read() throws IOException{inp = new StringTokenizer(br.readLine());}
    public static int nextInt(){return Integer.parseInt(inp.nextToken());}
    public static long nextLong(){return Long.parseLong(inp.nextToken());}
    public static double nextDouble(){return Double.parseDouble(inp.nextToken());}
    public static String next(){return inp.nextToken();}
    
}
