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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 a
 e
 a
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 y
 b
 aa
 a
 ab
 z
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 a

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
    static boolean dfs(char c, String s, String txt, int n )
    {
        if(n == 1)
        {
            if(txt.indexOf(s + c) < 0)
            {
                System.out.println(s + c);
                return true;
            }
            return false;       
        }
        for (char  r = 'a'; r <= 'z'; ++r) 
            if(dfs(r, s + c, txt, n - 1))               
                return true;
        return false;
        
    }
    public static void main(String[] args) throws NumberFormatException, IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));       
        int n = Integer.parseInt(in.readLine());
        String  cad = "" ;
        for (int i = 0; i < n; ++i) 
            cad += in.readLine()+" ";

        int i = 1;
        while(true)
        { 
            String s = "";
            for (char  r = 'a'; r <= 'z'; ++r)
                if(dfs(r, s, cad, i))
                    return;     
            i++;
        }       
    }
}
