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 2
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 5
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 2
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 1
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 10
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 1
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import java.io.*;
import java.util.*;
public final class new_prob
{
	static Scanner sc=new Scanner(System.in);
	static PrintWriter out=new PrintWriter(System.out);
	
	public static void main(String args[]) throws Exception
	{
		int n=sc.nextInt();
		boolean[] v=new boolean[123];
		boolean[][] a=new boolean[123][123];
		for(int i=0;i<n;i++)
		{
			char[] arr=sc.next().toCharArray();
			for(int j=0;j<arr.length;j++)
			{
				if(!v[arr[j]])
				{
					v[arr[j]]=true;
				}
				if(j>0)
				{
					a[arr[j-1]][arr[j]]=true;
				}
			}
		}
		String ans="";
		for(int i=97;i<=122;i++)
		{
			if(!v[i])
			{
				ans=((char)i)+"";
				break;
			}
		}
		if(ans.length()==0)
		{
			outer:
			for(int i=97;i<=122;i++)
			{
				for(int j=97;j<=122;j++)
				{
					if(!a[i][j])
					{
						ans=(char)i+""+(char)j;
						break outer;
					}
				}
			}
		}
		out.println(ans);
		out.close();
	}
}