#include <algorithm>
#include <iostream>
#include <cstdio>
#include <map>
#include <numeric>
#include <cmath>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <complex>
#include <string.h>
#include <unordered_set>
#include <unordered_map>
#include <bitset>
#include <iomanip>
#include <sys/time.h>
#include <random>
using namespace std;

#define endl '\n'
#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define UNIQ(v) (v).erase(unique((v).begin(), (v).end()), (v).end())

typedef long long ll;
typedef long double ld;
typedef pair<int, int> P;
typedef complex<double> comp;
typedef vector< vector<ld> > matrix;
struct pairhash {
public:
    template<typename T, typename U>
    size_t operator()(const pair<T, U> &x) const {
	size_t seed = hash<T>()(x.first);
	return hash<U>()(x.second) + 0x9e3779b9 + (seed<<6) + (seed>>2);
    }
};
const int inf = 1e9 + 9;
const ll mod = 1e9 + 7;
const double eps = 1e-8;
const double pi = acos(-1);

int n;
int a[110];

double dp[110][110][110];

double calc(int n0, int n1, int n2) {
    if (dp[n0][n1][n2] < 0) {
        double res = 0;
        if (n2 > 0) res += (double)n2/n * (1.0 + calc(n0, n1+1, n2-1));
        if (n1 > 0) res += (double)n1/n * (1.0 + calc(n0+1, n1-1, n2));
        if (n0 > 0) res += (double)n0/n * (1.0 + calc(n0-1, n1, n2));
        int k = n - n0 - n1 - n2;
        res += (double)k/n;
        res *= (double)n/(n-k);
        dp[n0][n1][n2] = res;
    }
    return dp[n0][n1][n2];
}

double solve() {
    int num[3] = {0, 0, 0};
    for (int i = 0; i < n; i++) {
        if (a[i] < 3)
            num[2-a[i]]++;
    }
    for (int i = 0; i <= n; i++)
        for (int j = 0; j <= n; j++)
            for (int k = 0; k <= n; k++)
                dp[i][j][k] = -1;
    dp[0][0][0] = 0;
    return calc(num[0], num[1], num[2]);
}

void input() {
    cin >> n;
    for (int i = 0; i < n; i++) cin >> a[i];
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(15);

    input();
    cout << solve() << endl;
}