using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		if(N==2){
			Console.WriteLine(2);
			return;
		}
		double d=2;
		for(int n=3;n<=N;n++){
			d+=2.0/(double)n;
			for(int i=1;i<n;i++){
				for(int j=1;j<n;j++){
					if(i==j)continue;
					double e=(double)i*j/(double)(n-1)/n;
					d+=e;
				}
			}
		}
		Console.WriteLine(d);
		
	}
	int N;
	public Sol(){
		N=ri();
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}