#include <bits/stdc++.h>
using namespace std;
using R=long double;
int main(void){
	R x;
	cin >> x;
	R ans=0.0;

	const int limit=10000000.0;
	
	R cur=x+limit;
	while(cur>x){
		ans+=1.0/cur/cur;
		cur-=1.0;
	}

	R offset=0.0;
	cur=limit;
	while(cur>=1.0){
		offset+=1.0/cur/cur;
		cur-=1.0;
	}

	ans+=acos(-1.0)*acos(-1.0)/6.0-offset;

	cout.precision(20);
	cout <<  fixed << ans << endl;
	return 0;
}