#include<iostream>
using namespace std;

int main(){
	int n;
	cin >> n;
	
	double sum = 0;
	double a = 2, b = 1;
	
	for(int i = 0; i < n; i++){
		sum += (a / b);
		double tmp = b;
		b = a;
		a = tmp + a;
	}
	
	printf("%.2f", sum);
	return 0;
}