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 30
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 2
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 5
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 2
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 1
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import java.io.IOException;
import java.util.Scanner;

/**
 *
 * @author Oreste
 */
public class Main {

    static int[][] arr;
    public static int Distancia(int N1,int N2,int NA){
        return Math.min(arr[N1][N2], arr[N1][NA]+arr[NA][N2]);
    }
    public static void main(String[] args) throws IOException {
       Scanner entrada=new Scanner(System.in);
       int num=entrada.nextInt();
       String cad="";
       while(num-->0){
           String cad1=entrada.next();
           cad+=cad1+" ";
       }
       String Result="";
       String Result1="";
       int cont=0,cont1=1;
       boolean esta=true;
       while(true){
           for(int i=0;i<26;i++){
               cont=cont1;
               Result1=new String();
               while(cont > 0){
        cont--;
        Result1 = ""+(char)(cont % 26 + 'a')+Result1;
        cont/=26;
                }
               cont1++;
               if(!cad.contains(Result1)){
                   esta=false;
                   System.out.println(Result);
                   break;
               }
               
           }          
           if(!esta)break;
       }
        System.out.println(Result1);
    }
}