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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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 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
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 e
 a
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 y
 b
 aa
 a
 ab
 z
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import java.io.*;
import java.math.*;
import java.util.*;

public class Task {
	
	private static String ans = "";
	private static String[] a;
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		/**
		 * ########################
		 * Solution
		 */
	
		int n = sc.nextInt();
sc.nextLine();
		a = new String[n];
		
		for(int i = 0; i < n; i++)
			a[i] = sc.nextLine();
		
		find(ans);
				
		out.println(ans);
		/**
		 * Solution
		 * ########################
		 */	
		
		out.flush();
	}
	
	private static boolean check(String s)
	{
		for(int i = 0; i < a.length; i++)
		{
			if(a[i].contains(s))
				return false;
		}
		return true;
	}
	
	private static void find(String s)
	{
		for(int i = 'a'; i <= 'z'; i++)
		{
			if(check(s+((char)i)))
			{
				ans = s+((char)i);
				return;
			}
		}
		for(int i = 'a'; i <= 'z'; i++)
		{
			if(ans.equals(""))
				find(s+'a');
		}
	}
}