#include <iostream>
#include <iomanip> 
using namespace std;

int main() {
	int k;
	long double val = 0;
	cin >> k;
	for (int n = 1; n <= k; n++){
		val += 1.0/((3*n - 2)*(3*n + 1)); 
	}
	cout << setprecision(15) <<val;
	return 0;
}