#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;
typedef pair<ll,ll> l_l;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
const int INF=1001001000;
const int mINF=-1001001000;
const ll LINF=1010010010010010000;
template<class T> inline bool chmin(T& a, T b) {
if (a > b) {
a = b;
return true;
}
return false;
}
template<class T> inline bool chmax(T& a, T b) {
if (a < b) {
a = b;
return true;
}
return false;
}
//グリッド：（典型）dp,dfs,bfs,最短経路,その他
int cnt[4];
int n,a;
double dp[110][110][110];
double dfs(int x,int y,int z){
    if(dp[x][y][z]>=0) return dp[x][y][z];
    dp[x][y][z]=n*1.0/(x+y+z);
    if(x>0) dp[x][y][z]+=dfs(x-1,y,z)*x/(x+y+z);
    if(y>0) dp[x][y][z]+=dfs(x+1,y-1,z)*y/(x+y+z);
    if(z>0) dp[x][y][z]+=dfs(x,y+1,z-1)*z/(x+y+z);
    return dp[x][y][z];
}
int main(){
    cin>>n;
    rep(i,110){
        rep(j,110){
            rep(k,110){
                dp[i][j][k]=-1;
            }
        }
    }
    rep(i,n){
        cin>>a;
        if(a==0) cnt[3]++;
        else if(a==1) cnt[2]++;
        else if(a==2) cnt[1]++;
    }
    dp[0][0][0]=0.0;
    cout <<fixed<<setprecision(10);
    cout << dfs(cnt[1],cnt[2],cnt[3]) << endl;
    return 0;
}