#include <iostream>
using namespace std;
int main() {
	float n, sum = 0, m = 1, c = 2;
	cin >> n;
	for (int i = 0; i < n; i++) {
		sum += c / m;
		int temp = c;
		c += m;
		m = temp;
	}
	printf("%.2f",sum);
	return 0;
}