3
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vpojopbwbwbswdu
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 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
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 5
splt
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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kopsdfgiopjipw
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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xcn
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x
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ci
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jos
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ge
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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;
import java.util.LinkedList;
import java.util.Queue;

public class Main {
    static Scanner in = new Scanner(System.in);
    static int n;
    static String[] S;
    
    public static boolean can(String s){
        for(int i = 0; i < n; i++)
            if(S[i].contains(s)) return false;
        return true;
    }
    
    public static void solve(){
        Queue<String> cola = new LinkedList<String>();
        String top;
        for(char c = 'a'; c <= 'z'; c++) cola.add(""+c);
        do{
            top = cola.poll();
            for(char c = 'a'; c <= 'z'; c++) cola.add(top+c);
        }while(!can(top));
        System.out.println(top);
    }
    
    public static void main(String[] args){
        n = in.nextInt();
        S = new String[n];
        for(int i = 0; i < n; i++) S[i] = in.next();
        solve();
    }
}