#include<bits/stdc++.h>
// Including Policy Based DS
//#include <ext/pb_ds/assoc_container.hpp>
// Including tree_order_statistics_node_update
//#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
//using namespace __gnu_pbds;

#define LL long long
#define ULL unsigned long long
LL INF=10000000000000001LL;
#define PI (2.0*acos(0.0))

typedef pair<int,int> pii;
typedef pair<LL,LL> pll;
typedef vector<int> vi;
typedef vector<LL> vl;
typedef vector<pii> vii;
typedef vector<pll> vll;

//typedef tree<
//int,
//null_type,
//less<int>,
//rb_tree_tag,
//tree_order_statistics_node_update>
//ordered_set;

#define READ 		freopen("in.txt","r",stdin)
#define WRITE  		freopen("out.txt","w",stdout)
#define BOOST		ios_base::sync_with_stdio(false);cin.tie(NULL)
#define sf 			scanf
#define pf 			printf
#define F  			first
#define S  			second
#define pb 			push_back
#define NL			pf("\n")
#define ALL(v)		v.begin(),v.end()
#define si(x)		sf("%d",&x);
#define sii(x,y)	sf("%d%d",&x,&y);
#define siii(x,y,z)	sf("%d%d%d",&x,&y,&z);
#define sl(x)		sf("%lld",&x);
#define sll(x,y)	sf("%lld%lld",&x,&y);
#define slll(x,y,z)	sf("%lld%lld%lld",&x,&y,&z);
#define mem(x,y) 	memset(x,y,sizeof x )
#define ch 			printf("Came Here!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n")
#define SV(v)		sort(v.begin(),v.end())
#define SVR(v,cmp)	sort(v.begin(),v.end(),cmp)
#define FOR(i,n)	for(int i=0;i<n;i++)
#define cnd			tree[idx]
#define lc			tree[idx*2]
#define rc			tree[idx*2+1]
#define lnd 		(2*idx),(b),((b+e)/2)
#define rnd 		((2*idx)+1),(((b+e)/2)+1),(e)

// graph direction array [4]
LL X[4]={0,0,-1,1};
LL Y[4]={1,-1,0,0};
// graph direction array [8]
//int X[8]={0,0,1,-1,-1,1,1,-1};
//int Y[8]={1,-1,0,0,-1,-1,1,1};
// Bishop direction array [8]
//int X[8]={0,0,1,-1,-1,1,1,-1};
//int Y[8]={1,-1,0,0,-1,-1,1,1};
// Knight Direction Array
//int KX[8] = {1,1,2,2,-1,-1,-2,-2};
//int KY[8] = {2,-2,1,-1,2,-2,1,-1};

bool Check(int N,int pos){return (bool)(N & (1<<pos));} // check bit
int Set(int N,int pos){	return N=N | (1<<pos);}// set bit

const LL mod=1000000007LL;
LL modMul(LL a, LL b){

    LL ans = 0;
    a = a % mod;
    while (b > 0){
        if ( b % 2 )ans = (ans%mod+ a%mod) % mod;
        a = (a%mod * 2%mod) % mod;
        b /= 2;
    }
    return ans % mod;
}

LL powerMod(LL a,LL b){
	if(b==0)return 1%mod;
	LL x=powerMod(a,b/2);
	x = (x%mod*x%mod)%mod;
	if(b%2==1)x = (x%mod*a%mod)%mod;
	return x%mod;
}

LL lcm( LL a,LL b ){
	LL g = __gcd(a,b);
	return (a*b)/g;
}
const int mx = 100005;
//-------------------------------------------------------------------------------


double dp[302][302][302];
bool vis[302][302][302];
int n;
double f(int one,int two,int three){

	int zero = n-one-two-three;
	if(one<0||two<0||three<0)return 0;
	if( zero<0 )return 0;
	if( zero==n )return 0;
	if( vis[one][two][three] )return dp[one][two][three];
	vis[one][two][three] = 1;
	double tot = 1.0 + (one/(double)n)*f(one-1,two,three) + (two/(double)n)*f(one+1,two-1,three) + (three/(double)n)*f(one,two+1,three-1);
    double x = (n*tot)/(n-zero);
    return dp[one][two][three] = x;

}



int main(){
//  READ;
//  WRITE;
//	BOOST;

	int m,k,t;
	int x=0,y=0,z=0;
	si(n);
	FOR(i,n){
        si(k);
        if(k==1)x++;
        else if(k==2)y++;
        else if(k==3)z++;
	}

	pf("%.12f\n",f(x,y,z));






    return 0;
}



Measured memory_bytes: 241920