#include <iostream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = uint32_t;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

template <class T, class U>
vector<T> make_v(U size, const T& init){ return vector<T>(static_cast<size_t>(size), init); }

template<class... Ts, class U>
auto make_v(U size, Ts... rest) { return vector<decltype(make_v(rest...))>(static_cast<size_t>(size), make_v(rest...)); }

template<class T> void chmin(T &a, const T &b){ a = (a < b ? a : b); }
template<class T> void chmax(T &a, const T &b){ a = (a > b ? a : b); }

int main() {
    int n;
    cin >> n;
    vector<int> cnt(4);
    for (int i = 0; i < n; ++i) {
        int x;
        scanf("%d", &x);
        cnt[min(x, 3)]++;
    }
    auto dp = make_v(n+1, n+1, n+1, 0.0);
    dp[cnt[0]][cnt[1]][cnt[2]]++;
    double ans = 0;
    for (int i = n; i >= 0; --i) {
        for (int j = n-i; j >= 0; --j) {
            for (int k = n-i-j; k >= 0; --k) {
                if(i+j+k == 0) continue;
                ans += dp[i][j][k]*(1.0*n/(i+j+k)-1);
                if(i && j != n) {
                    dp[i-1][j+1][k] += 1.0*i/(i+j+k)*dp[i][j][k];
                    ans += 1.0*i/(i+j+k)*dp[i][j][k];
                }
                if(j && k != n){
                    dp[i][j-1][k+1] += 1.0*j/(i+j+k)*dp[i][j][k];
                    ans += 1.0*j/(i+j+k)*dp[i][j][k];
                }
                if(k) {
                    dp[i][j][k-1] += 1.0*k/(i+j+k)*dp[i][j][k];
                    ans += 1.0*k/(i+j+k)*dp[i][j][k];
                }
            }
        }
    }
    printf("%.10lf\n", ans);
    return 0;
}