#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n - 1; i >= 0; --i)
#define FOR(i, m, n) for (int i = m; i < n; ++i)
#define FORR(i, m, n) for (int i = m; i >= n; --i)
#define ALL(v) (v).begin(),(v).end()
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const ll INF=1LL<<60;
const int inf=(1<<30)-1;
const int mod=1e6+7;
int dx[8]={1,0,-1,0,-1,-1,1,1};
int dy[8]={0,1,0,-1,-1,1,-1,1};
map<vector<double>,double> mp;
double n;
double f(vector<double> c){
    if(mp.count(c)) return mp[c];
    if(c[3]==n) return mp[c]=0;
    double res=0;
    vector<double> d=c;
    if(c[0]){
        c[0]--,c[1]++;
        res+=(f(c)+1.)*d[0]/n;
        c[0]++,c[1]--;
    }
    if(c[1]){
        c[1]--,c[2]++;
        res+=(f(c)+1)*d[1]/n;
        c[1]++,c[2]--;
    }
    if(c[2]){
        c[2]--,c[3]++;
        res+=(f(c)+1)*d[2]/n;
        c[2]++,c[3]--;
    }
    return mp[c]=res*n/(n-d[3])+d[3]/(n-d[3]);
}
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n;
    vector<int> a(n);
    vector<double> c(4);
    REP(i,n){
        cin >> a[i];
        chmin(a[i],3);
        c[a[i]]++;
    }
    cout << fixed << setprecision(10) << f(c) << endl;
}