package 練習;
import java.util.Date;
import java.util.Scanner;
public class main{
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		double n=sc.nextDouble();
		long exec_time=new Date().getTime();
		double prd=0;
		double e_prd=0;
		double e_cost=1;
		for(double i=2;i<=n;i++){
			System.out.println("prd="+prd);
			if(i>2)
				e_prd=prd/((i-1)*(i-2));//積の平均
			e_cost+=2/i+e_prd*(i-2)/i;
			prd+=(i-1)*i*i;//1~iまでの２数の積の和
		}
		
		System.out.println(e_cost);
		System.err.println(new Date().getTime()-exec_time+"ms");

	}
}