3
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rydkiwnugwddcgcrng
 1
z
 30
b
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f
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j
k
p
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a
c
w
z
z
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 5
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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kjexeesevrlowxhghq
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z
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jrsrvsvbaiumlmkptn
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szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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itaubd
xcn
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x
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ge
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wnjwjrf
t
bq
bw
 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.ObjectInputStream.GetField;
import java.nio.file.Files;
import java.util.HashSet;
import java.util.StringTokenizer;

public class Main
{
	private static BufferedReader in;
	private static BufferedWriter out;
	private static HashSet<String> takenNames;
	private static char[] chars;

	public static void main(String[] args) throws NumberFormatException, IOException
	{
		// streams
		boolean file = false;
		if (file)
			in = new BufferedReader(new FileReader("input.txt"));
		else
			in = new BufferedReader(new InputStreamReader(System.in));
		out = new BufferedWriter(new OutputStreamWriter(System.out));
		StringTokenizer tok;

		// store all previous substrings
		takenNames = new HashSet<String>();
		int n = Integer.parseInt(in.readLine());
		for (int i = 0; i < n; i++)
		{
			String str = in.readLine().trim();
			for (int j = 0; j < str.length(); j++)
				for (int j2 = j + 1; j2 <= str.length(); j2++)
					takenNames.add(str.substring(j, j2));
		}
			
		// make the name with the shortest length
		for (int l = 1; l <= 21; l++)
		{
			// start with a string of all as
			chars = new char[l];
			for (int i = 0; i < l; i++)
				chars[i] = 'a';

			// keep incrementing it
			 boolean canIncrement = true;
			while (canIncrement)
			{
				// check if it's a new String
				String str = new String(chars);
				if (!takenNames.contains(str))
				{
					System.out.println(str);
					return;
				}

				// increment it
				canIncrement = increment(l);
			}

		}

	}

	private static boolean increment(int l)
	{
		// try incrementing from right to left
		for (int i = l - 1; i >= 0; i--)
			if (chars[i] != 'z')
			{
				chars[i] += 1;
				return true;
			} else
				chars[i] = 'a';

		return false;
	}

}