Tower of Hanoi iteration Count

import java.util.*;

public class HanoiTowerCount

{

                static double hanoi(int n)

        {

            if (n==0)

                return 1;

            else

                return(2*hanoi(n-1));

                }

                public static void main(String[] args)

    {

                                Scanner in=new Scanner(System.in);

                                System.out.println("Enter no. of rings please");

                                int n=in.nextInt();

                                double ans=hanoi(n);

                                System.out.println(--ans);

    }

}