#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>
#include<set>

#define rep(i,n) for(int (i) = 0; i < (n); i++)
using namespace std;
using ll = long long;
using ull = unsigned long long;

const int inf = 1e9 + 7;

int main(){ 
	cout << fixed << setprecision(15);
	int k;
	cin >> k;
	long double sum = 0;
	for(int i = 1; i <= 1000000; i++){
		long double now = (long double)1/((long double)i*((long double)i + (long double)k));
		sum += now;
	}
	cout << sum << endl;
	return 0;    
}