#include<iostream>
#include<stdio.h>
using namespace std;
int n;
double a[31]={0,2,3},b[31]={0,1,2},sum;
int main(){
	cin>>n;
	if(1<=n)sum=2;
	if(2<=n)sum=3.5;
	for(int i=3;i<=n;i++){
		a[i]=a[i-1]+a[i-2];
		b[i]=b[i-1]+b[i-2];
		sum+=a[i]/b[i];
	}
	printf("%.4lf",sum);
	return 0;
}