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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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import	java.util.*;
public	class	Main{
	public	static	void	main(String[]args){
		boolean[]F=new	boolean[26],H[]=new	boolean[26][26];
		Scanner	cin=new	Scanner(System.in);
		String	z;
		for(int	n,m;cin.hasNextInt();Arrays.fill(F,false)){
			for(n=cin.nextInt();0<n--;F[z.codePointAt(0)-'a']=true)
				for(m=(z=cin.next()).length();0<--m;F[z.codePointAt(m)-'a']=true)
					H[z.codePointAt(m-1)-'a'][z.codePointAt(m)-'a']=true;
			if(F.length==(m=Algorithm.find(F,false))){
				for(n=0;H[n].length==(m=Algorithm.find(H[n],false));++n);
				System.out.println((char)('a'+n)+""+(char)('a'+m));
			}else	System.out.println((char)('a'+m));
			for(boolean[]E:H)	Arrays.fill(E,false);
		}
	}
}
final	class	Algorithm{
	private	Algorithm(){}
	static	int	find(boolean[]F,boolean	f){return	find(F,0,F.length,f);}
	static	int	find(boolean[]F,int	l,int	r,boolean	f){
		for(;r>l&&f!=F[l];++l);
		return	l;
	}
}
