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

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

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.*;
public class code {


    public static void main(String[] args) throws IOException {
     Scanner k=new Scanner(System.in);
     int x=k.nextInt();
     String[] array=new String[x];
     for (int i = 0; i < array.length; i++) {
        array[i]=k.next();
    }
     String problem="a";
     boolean flag=true;
     while (flag) {
        for (int i = 0; i < array.length; i++) {
            if (array[i].contains(problem)) {
                flag=false;
                break;
            }
        }
         
        if (!flag) {
            flag=true;
            problem=findnext(problem);
        }
        else {
            flag=false;
        }
         
    }
     System.out.println(problem);
     
    }
    public static String findnext(String prob){
        String next="";
        if (prob.charAt(prob.length()-1)!='z') {
            char last=(char) (prob.charAt(prob.length()-1)+1);
            for (int j = 0; j < prob.length(); j++) {
                if (prob.length()-1==j) {
                    next+=last;
                }
                else {
                    next+=prob.charAt(j);
                }
            }
        }

        else {
     int i;
            for ( i = prob.length()-1; i >=0; i--) {
                if (prob.charAt(i)!='z') {
                break;  
                }
            }
            if (i<0) {
                for (int j = 0; j <= prob.length(); j++) {
                    next+='a';
                }
            }
            else {
                char x=(char)(prob.charAt(i)+1);
                for (int j = 0; j < prob.length(); j++) {
                    if (i==j) {
                        next+=x;
                    }
                    else if (j>i) {
                        next+='a';
                    }
                    else {
                        next+=prob.charAt(j);
                    }
                }
            }
        }
        
        
        return next;
    }
    

    
}
