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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 2
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 1
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 30
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 1
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 a

import java.util.*;

public class namingproblems
{

	public static void main(String args[])
	{
	
		int n,i;
		String str[];
		
		Scanner s = new Scanner(System.in);
		n = s.nextInt();
		
		str = new String[n];
		
		for(i=0;i<n;++i)
		str[i] = s.next();
		String ss = "";
		
		int maxlen = 0;
		
		for(i=0;i<n;++i)
		if(str[i].length()>maxlen) maxlen = str[i].length();
		
		recfunc(ss,str,maxlen, 1);
		
		
	}
	
	
	public static void recfunc(String ss,String strings[], int len, int ctr)
	{
	char i;
	
	if(ctr==len)
	return ;
	
	
	for(i='a';i<='z';++i)
	{
	if(check((ss+i),strings))
	{
	System.out.println(ss+i);
	System.exit(0);
	}
	}


	for(i='a';i<='z';++i)
	{
	ctr++;
		recfunc(ss+i, strings, len, ctr+1);
		
	}
	
	
	}
	
	
	
	public static boolean check(String a, String strings[])
	{
	int i, n = strings.length;
	
		for(i=0;i<n;++i)
		{
		if(strings[i].indexOf(a) !=-1)
		return false;
		}
		
		return true;
	}
	
	
}
