#include <bits/stdc++.h>
#define be(v) (v).begin(),(v).end()
#define pb(q) push_back(q)
typedef long long ll;
using namespace std;
const ll mod=1000000007;
#define doublecout(a) cout<<fixed<<setprecision(15)<<a<<endl;
double dp[301][301][301];
int n;
double solve(int i,int j,int k){
	if(dp[i][j][k]>=0)return dp[i][j][k];
	if(!(i|j|k))return 0.0;
	double res=0.0;
	if(i)res+=solve(i-1,j,k)*i;
	if(j)res+=solve(i+1,j-1,k)*j;
	if(k)res+=solve(i,j+1,k-1)*k;
	res+=n;
	res/=(i+j+k);
	return dp[i][j][k]=res;
}
int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);
    cin>>n;
    int a,c[4]={0};
    for(int i=0;i<n;i++){
    	cin>>a;
    	//c[a]++;
    	if(a<3)c[3-a]++;
    }
    memset(dp,-1,sizeof(dp));
    doublecout(solve(c[1],c[2],c[3]));
    return 0;
}