#include<bits/stdc++.h>
#define For(i,x,y) for (int i=(x);i<=(y);i++)
#define FOR(i,x,y) for (int i=(x);i<(y);i++)
#define Dow(i,x,y) for (int i=(x);i>=(y);i--)
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define ep emplace_back
#define siz(x) ((int)x.size())
#define all(x) x.begin(),x.end()
#define fil(a,b) memset(a,b,sizeof a)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pa;
typedef pair<ll,ll> PA;
typedef vector<int> poly;
inline ll read(){
	ll x=0,f=1;char c=getchar();
	while ((c<'0'||c>'9')&&(c!='-')) c=getchar();
	if (c=='-') f=-1,c=getchar();
	while (c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
	return x*f;
}

const int N = 2e5+10;
int n,c[10];
ll fac[22];

inline double calc(int x,int y){
	double ans=0;
	FOR(i,0,1<<(2*(x+y))) if (__builtin_popcount(i)==2*x){
		int f1=0,f2=0;
		FOR(j,0,2*(x+y)) if (i>>j&1){
			if (!f1){
				if (f2) ans+=1;
			} else {
				if (f2) ans+=0.5;
			}
			f1^=1;
		} else {
			if (!f2){
				if (f1) ans+=1;
			} else {
				if (f1) ans+=0.5;
			}
			f2^=1;	
		}
	}
	ans=ans*(1.0*fac[2*x]*fac[2*y]/fac[2*(x+y)]);
	return ans;
}

int main(){
	n=read();
	For(i,1,n) c[read()]++;
	fac[0]=1;
	For(i,1,20) fac[i]=fac[i-1]*i;
	double ans=0;
	For(i,1,5){
		ans+=calc(i,i)*(1ll*(c[i]-1)*c[i]/2);
		ans+=c[i]*i*3;
		For(j,i+1,5) ans+=calc(i,j)*c[i]*c[j];
	}
	printf("%0.12lf\n",ans);
}