#include <stdio.h>
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000005


int main(){
	
	double k;
	cin>>k;
	double ans = 0.0;
	rep(i,10000000){
		ans += 1.0 / ((i+1)*(i+1+k));
	}
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
    return 0;
}