#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include "debug.hpp"
#else
#define debug(...) 1
#endif

int n;
vector<int> a;
vector<vector<vector<bool>>> seen;
vector<vector<vector<double>>> memo;

double solve(int c1, int c2, int c3) {
    if (seen[c1][c2][c3]) {
        return memo[c1][c2][c3];
    }
    seen[c1][c2][c3] = true;
    if (c1 == 0 && c2 == 0 && c3 == 0) {
        return 0.0;
    }
    double ret = 1;
    if (c1 >= 1) {
        ret += solve(c1 - 1, c2, c3) * (double) c1 / n;
    }
    if (c2 >= 1) {
        ret += solve(c1 + 1, c2 - 1, c3) * (double) c2 / n;
    }
    if (c3 >= 1) {
        ret += solve(c1, c2 + 1, c3 - 1) * (double) c3 / n;
    }
    if (n - (c1 + c2 + c3) >= 1) {
        ret *= (double) n / (c1 + c2 + c3);
    }
    return memo[c1][c2][c3] = ret;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n;
    a.resize(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    seen.resize(n + 1, vector<vector<bool>>(n + 1, vector<bool>(n + 1, false)));
    memo.resize(n + 1, vector<vector<double>>(n + 1, vector<double>(n + 1, 0)));
    int c1 = 0, c2 = 0, c3 = 0;
    for (int i = 0; i < n; i++) {
        int x = max(0, 3 - a[i]);
        if (x == 1) c1++;
        if (x == 2) c2++;
        if (x == 3) c3++;
    }
    double ans = solve(c1, c2, c3);
    cout << fixed << setprecision(15) << ans << '\n';
}