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 2
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 1
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import java.util.*;
import java.io.*;

public class Main
{

	static ArrayList<String> arr = new ArrayList<String>();

	public static void main(String[] args)
	throws Exception
	{
		BufferedReader reader = new BufferedReader(
			new InputStreamReader(System.in));

		PrintWriter writer = new PrintWriter(System.out);
		StringTokenizer sz;

		int n = Integer.parseInt(reader.readLine());

		for (int i=1;i <= n;++i)
			arr.add(reader.readLine());

		for (char ch='a';ch <= 'z';++ch)
		{
			if (testSub(ch + ""))
			{
				System.out.println(ch);
				return;
			}

		}

		for (char f='a';f <= 'z';++f)
			for (char s='a';s <= 'z';++s)
				if (testSub(f + "" + s))
				{
					System.out.println(f + "" + s);
					return;
				}


		writer.flush();

	}

	public static boolean testSub(String sub)
	{

		for (String el:arr)
			if (el.contains(sub))
				return false;

		return true;
	}


} 