3
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 1
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 30
b
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p
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a
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z
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d
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
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 5
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abcdefghijklmnopqrst
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 1
epkotfpkkrhhmuipmtdk
 10
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 5
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 2
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 1
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 30
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 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import java.util.*;
public class cf{
    public static void main(String args[]){
	Scanner sc=new Scanner(System.in);
	int n=sc.nextInt();
	String[] a=new String[n];
	String ans="";
	for(int i=0;i<n;i++)a[i]=sc.next();
	ArrayList<String> words=new ArrayList<>();
	for(char ch='a';ch<='z';ch++)words.add(""+ch);
	boolean found=false;
	while(!found){
	    for(String x:words){
		int i=0;
		for(;i<n;i++)
		    if(a[i].contains(x))break;
		if(i==n){
		    found=true;
		    ans=x;
		    break;
		}
	    }
	    if(found)break;
	    else words=getSubstrings(words);
	}
	System.out.println(ans);
    }
    private static ArrayList<String> getSubstrings(ArrayList<String> a){
	ArrayList<String> b=new ArrayList<>();
	for(String x:a)
	    for(char ch='a';ch<='z';ch++)
		b.add(x+ch);
	return b;
    }
}