#include <bits/stdc++.h>
using namespace std;

int N;
double memo[110][110][110];

double dfs(int a, int b, int c) {
    if (memo[a][b][c])
        return memo[a][b][c];
    if (a + b + c == 0)
        return 0;
    double ret = 0;
    if (a > 0)
        ret += dfs(a - 1, b + 1, c) * a / N;
    if (b > 0)
        ret += dfs(a, b - 1, c + 1) * b / N;
    if (c > 0)
        ret += dfs(a, b, c - 1) * c / N;
    ret += 1;
    ret *= (double)N / (a + b + c);
    return memo[a][b][c] = ret;
}

int main() {
    int A[110];
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }

    int cnt[3] = {};
    for (int i = 0; i < N; i++) {
        if (A[i] < 3)
            cnt[A[i]]++;
    }

    cout << fixed << setprecision(10) << dfs(cnt[0], cnt[1], cnt[2]) << endl;
}