3
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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
b

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 e
 a
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 aa
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 a

import java.util.*;

public class NewProblem278B
{
    public static void main(String[] args) 
    {
        // Set up scanner
        Scanner sc = new Scanner(System.in); 
        // System.out.println("Enter n"); 
        int n = sc.nextInt();

        ArrayList<String> t = new ArrayList<String>();
        for (int i=0; i<n; i++)
        {
            // System.out.println("Enter next string");
            t.add(sc.next());
        }

        // Loop over one letters
        for (char ch = 'a'; ch <= 'z'; ch++)
        {
            String next = String.valueOf(ch);
            boolean found = false;
            for (String str: t)
            {
                if (str.indexOf(next) >= 0)
                {
                    found = true;
                }
            }
            if (!found)
            {
                System.out.println(next);
                return;
            }
        }

        // Loop over two letters
        for (char ch1 = 'a'; ch1 <= 'z'; ch1++)
        {
            for (char ch2 = 'a'; ch2 <= 'z'; ch2++)
            {
                char[] arr = new char[2];
                arr[0] = ch1;
                arr[1] = ch2;
                String next = new String(arr);
                boolean found = false;
                for (String str: t)
                {
                    if (str.indexOf(next) >= 0)
                    {
                        found = true;
                        break;
                    }
                }
                if (!found)
                {
                    System.out.println(next);
                    return;
                }
            }
        }

        //  Loop over three letters--can't be longer!

        for (char ch1 = 'a'; ch1 <= 'z'; ch1++)
        {
            for (char ch2 = 'a'; ch2 <= 'z'; ch2++)
            {
                for (char ch3 = 'a'; ch3 <= 'z'; ch3++)
                {
                    char[] arr = new char[3];
                    arr[0] = ch1;
                    arr[1] = ch2;
                    arr[2] = ch3;
                    String next = new String(arr);
                    boolean found = false;
                    for (String str: t)
                    {
                        if (str.indexOf(next) >= 0)
                        {
                            found = true;
                            break;
                        }
                    }
                    if (!found)
                    {
                        System.out.println(next);
                        return;
                    }
                }
            }
        }
    }
}


        