#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
	double pi = 0.0;
	cin >> n;
	for (int i = 1; i <= n; i++){
		if (i % 2 != 0)
			pi = pi + 4.0 * 1 / (2 * i - 1);
		else
			pi = pi - 4.0 * 1 / (2 * i - 1);
	} 
	cout << fixed << setprecision(6) << pi;
	return 0;
}