#include <iostream>
#include <fstream>
#include <deque>
#include <cmath>

using namespace std;

double memo[501][501];

double f(int a, int b)
{
	if(memo[a][b]>-0.5)
	{
		//cerr<<"f("<<a<<" "<<b<<") mem="<<memo[a][b]<<endl;
		return memo[a][b];
	}
	double & ret = memo[a][b];


	if(a==0 && b==0) return ret=0.0;
	if(a==1 && b==0) return ret=0.0;
	if(a==0 && b==1) return ret=0.0;
	//if(a==2 && b==0) return ret=0.0;

#if 0
	// TODO 
	double x = 1.0 / (a+2*b) / (a+2*b -1) *2;
	cerr<<"x="<<x<<" a="<<a<<" b="<<b<<endl;
	double sum1 = 0;
	if(a>1       ) sum1 += a*(a-1)/2* (f(a-2,b)+1) *x;
	cerr<<"a,b,sum1a ("<<a<<" "<<b<<") "<<sum1<<endl;
	if(a>0 && b>0) sum1 += a*(2*b) * (f(a, b-1)+1) *x;
	cerr<<"a,b,sum1b ("<<a<<" "<<b<<") "<<sum1<<endl;
	if(b>0       ) sum1 += b* f(a,b-1) *x;
	cerr<<"a,b,sum1c ("<<a<<" "<<b<<") "<<sum1<<endl;
	if(b>1       ) sum1 += ((2*b)*(2*b-1)/2-b) * (f(a+2,b-2)+1) *x;
	cerr<<"a,b,sum1f ("<<a<<" "<<b<<") "<<sum1<<endl;


	cerr<<"a,b,sum1 ("<<a<<" "<<b<<") "<<sum1<<endl;
	if(a==0) return ret=sum1;

	double sum2 = 0;
	sum2 += (double)(1.0)/(a+2*b) *  f(a-1, b);
	sum2 += (double)(a-1)/(a+2*b) * (f(a-1, b)+1);
	if(b>0) sum2 += (double)(2*b)/(a+2*b) * (f(a+1, b-1)+1);

	cerr<<"a,b,sum2 ("<<a<<","<<b<<") "<<sum2<<endl;
	return ret= sum1 <? sum2;
#endif
	double sum1=0;
	double geta=(double)a/(a+2*b);
	if(a)sum1+=geta*f(a-1,b);

	double getb=1-geta;
	double getokb=getb*1/(a+2*b-1);
	if(b)sum1+=getokb*f(a,b-1);
	else getokb=0;
	double getba=getb*a/(a+2*b-1);
	if(a&&b)sum1+=getba*(f(a,b-1)+1);
	else getba=0;
	if(b>1)sum1+=(1-geta-getokb-getba)*(f(a+2,b-2)+1);

	return ret=sum1;
}

main()
{
	ifstream cin("I.txt");
	int n;
	for(int i=0; i<=500; i++) for(int j=0; j<=500; j++) memo[i][j]=-1.0;
	while(cin>>n && n)
	{
		printf("%.10lf\n",f(0,n/2));
	}
	return 0;
}