abc28d.java

import java.util.Scanner;
 
public class Main {
	public static int[][] maze =new int [100][];
	public static int[][] dist =new int [100][];
	public static int[][] dir  ={{-1,0},{1,0},{0,-1},{0,1}};
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		double N=sc.nextDouble();
		double K=sc.nextDouble();
		System.out.println((1+3*(N-K)+3*(K-1)+6*(K-1)*(N-K))/(N*N*N));
 
		sc.close();
	}
 
}