3
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 1
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 30
b
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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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a
 a
 e
 a
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 y
 b
 aa
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 ab
 z
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 l
 a

import java.util.*;
import java.io.*;

public class Main
{
    private static boolean has_bit(long a, int pos)
    {
        return (a & (1L << pos)) == 0;
    }
    public static void main(String[] args)
    {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        StringBuilder s = new StringBuilder();
        
        for (int i = 0; i < n; ++i)
        {
            String str = in.next() + " ";
            s.append(str);
        }

        char a = 'a';
        String str = s.toString();
        for (char c = 'a'; c <= 'z'; ++c)
        {
            if (str.indexOf(c) >= 0)
                continue;
            else
            {
                System.out.println(c);
                System.exit(0);
            }
        }
        char[] ck = new char[2];
        for (char c = 'a'; c <= 'z'; ++c)
        {
            ck[0] = c;
            for (char k = 'a'; k <= 'z'; ++k)
            {
                ck[1] = k;
                if (str.contains(String.valueOf(ck)))
                    continue;
                else
                {
                    System.out.println(ck);
                    System.exit(0);
                }
            }
        }

    }
}
    

