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 5
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 1
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//package round_171;

import java.io.PrintWriter;
import java.util.Scanner;

/**
 * User: v.manilo
 * Date: 2/28/13
 * Time: 7:02 PM
 */
public class NewTask {

    private boolean isUnique(String[] tasks, String test){
        for(String s : tasks){
            if(s.contains(test)){
                return false;
            }
        }
        return true;
    }

    private String getNext(String input){
        StringBuilder sb = new StringBuilder(input);
        if(input.equals(""))
            return "a";

        char last = input.charAt( input.length() - 1 );
        if(last < 'z'){
            last++;
            sb.setCharAt(input.length() - 1, last  );
        } else {
            sb.append('a');
        }

        return sb.toString();
    }

    public String getNewName(String[] tasks){

        String test = getNext("");
        do{
           if(isUnique(tasks, test)){
               return test;
           }
           test = getNext(test);
        }while (true);

    }

    public static void main(String[] argv){


        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int taskNum = in.nextInt();
        String[] tasks = new String[taskNum];
        for(int i = 0; i < taskNum; ++i){
            tasks[i] = in.nextLine();
        }

        NewTask task = new NewTask();

        out.println( task.getNewName(tasks) );
        out.flush();

    }

}
