#include<iostream>
#include<vector>
#include<string>
#include<cstring>
#include<algorithm>
#include<map>
#include<set>
#include<cmath>
#include<cassert>
#include<queue>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	long double x;
	cin >> x;
	long double ans = 3.333e-9;
	for (int i = 300000000; i >= 1; i--) {
		double tmp = 1/(x+i);
		ans += tmp*tmp;
	}
	printf("%.12Lf\n", ans);
	return 0;
}