#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;

int main() {
	int n;
	cin >> n;
	double arr1[1000], arr2[1000];
	arr1[0] = 2, arr1[1] = 3;
	arr2[0] = 1, arr2[1] = 2;
	double res = 0;
		for (int i = 2;i < n;i++) {
			arr1[i] = arr1[i - 1] + arr1[i - 2];
			arr2[i] = arr2[i - 1] + arr2[i - 2];
		}
	for (int i = 0;i < n;i++) {
		res += (arr1[i] / arr2[i]);

	}

	printf("%.2lf", res);
	return 0;
}