import java.util.*;
import java.lang.*;
import java.io.*;
 
class Ideone
{
	public static void main (String[] args) {
		double x = 0;
		Scanner in=new Scanner(System.in);
		int n = in.nextInt();
		for(int i = 1; i <= n; i++)	{ 
			x +=1/(i*i*i);
		}
 		System.out.println(x);
	}
}