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.*;
import java.util.Map.Entry;
import java.io.*;
import java.awt.Point;
import java.math.BigInteger;

import static java.lang.Math.*;

public class Codeforces_Solutions implements Runnable{
    
    final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
    
    BufferedReader in;
    PrintWriter out;
    StringTokenizer tok = new StringTokenizer("");
    
    void init() throws FileNotFoundException{
        if (ONLINE_JUDGE){
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
        }else{
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");
        }
    }

    String readString() throws IOException{
        while(!tok.hasMoreTokens()){
            try{
                tok = new StringTokenizer(in.readLine());
            }catch (Exception e){
                return null;
            }
        }
        return tok.nextToken();
    }
    
    int readInt() throws IOException{
        return Integer.parseInt(readString());
    }
    
    long readLong() throws IOException{
        return Long.parseLong(readString());
    }
    
    double readDouble() throws IOException{
        return Double.parseDouble(readString());
    }
    
    public static void main(String[] args){
        new Thread(null, new Codeforces_Solutions(), "", 128 * (1L << 20)).start();
    }
    
    long timeBegin, timeEnd;

    void time(){
        timeEnd = System.currentTimeMillis();
        System.err.println("Time = " + (timeEnd - timeBegin));
    }
    
    long memoryTotal, memoryFree;
    

    void memory(){
        memoryFree = Runtime.getRuntime().freeMemory();
        System.err.println("Memory = " + ((memoryTotal - memoryFree) >> 10) + " KB");
    }
    
    void debug(Object... objects){
        if (DEBUG){
            for (Object o: objects){
                System.err.println(o.toString());
            }
        }
    }
    
    public void run(){
        try{
            timeBegin = System.currentTimeMillis();
            memoryTotal = Runtime.getRuntime().freeMemory();
            init();
            solve();
            out.close();
            time();
            memory();
        }catch (Exception e){
            e.printStackTrace(System.err);
            System.exit(-1);
        }
    }
    
    
    boolean DEBUG = false;
    
    class Point {
        int x;
        int y;
        
        public Point(int x, int y) {
            this.x=x;
            this.y=y;
        }
        
    }
    
    
    void solve() throws IOException{
     
         int n = readInt();
         String[] s = new String[n];
         for (int i=0; i<n; i++) s[i]=readString();
         char[] symbol = new char[28];
         char temp='a';
         String ans=new String("aaaaaaaa");
         for (int i=1; i<=26; i++) symbol[i]=temp++;
         for (int a=1; a<=26; a++)
             for (int b=0; b<=26; b++)
                 for (int c=0; c<=26; c++)
                     for (int d=0; d<=26; d++){
                         StringBuilder t = new StringBuilder();
                         t.append(symbol[a]);
                         if (b>0) t.append(symbol[b]);
                         if (c>0) t.append(symbol[c]);
                         if (d>0) t.append(symbol[d]);
                         boolean f = true;
                         //out.println(t.toString());
                         for (int i=0; (i<n); i++) {
                             if (s[i].contains(t.toString())) f=false;
                         }
                         if (f) {
                             if (t.toString().length()<ans.length()) ans=t.toString();
                         }
                     }
         out.print(ans);
    }
}