#include "iostream"
#include "iomanip"
#include "math.h"
using namespace std;

long double N;
long double ans;

int main() {
	cin >> N;
	for (long double i = 1; i < 100000; i+=1) {
		ans += (long double)1 / (i + N) / (i + N);
	}
	cout << setprecision(20) << ans << "\n";
	return 0;
}