#include<cstdio>
#include<cstring>
#include<algorithm>
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
using namespace std;
const int N=5e5+5;
double f[15][15];
double g[15][15];
int a[N],cnt[N];
double calc(int x,int y)
{
	if(!y)return 0;
	if(!x)return 1.0;
	else return 0.5;
}
double get(int n,int m)
{
	rep(i,0,n+1)rep(j,0,m+1)f[i][j]=0.0;f[0][0]=1.0;
	double tot=0;
	rep(i,0,n)rep(j,0,m)
	{
		int w1=n-i,w2=m-j;
		if(!w1 && !w2)continue;
		f[i+1][j]+=1.0*w1/(w1+w2)*f[i][j];
		f[i][j+1]+=1.0*w2/(w1+w2)*f[i][j];
		if(i<n)tot+=1.0*1.0*w1/(w1+w2)*f[i][j]*calc(i%2,j%2);
	}
	return tot;
}
int main()
{
	rep(i,1,5)rep(j,1,5)g[i][j]=get(i*2,j*2);
	int n;scanf("%d",&n);
	double ans=0;
	rep(i,1,n)scanf("%d",&a[i]),cnt[a[i]]++;
	rep(i,1,n)ans+=a[i]*3.0;
	rep(i,1,5)rep(j,i+1,5)if(i!=j)ans+=1.0*cnt[i]*cnt[j]*(g[i][j]+g[j][i]);
	rep(i,1,5)ans+=1.0*cnt[i]*(cnt[i]-1)*g[i][i];
	printf("%.9lf\n",ans);
}