#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
#define rep(i, n) REP(i, 0, n)
#define REP(i, s, e) for (ll i = (s); i < (ll)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for (ll i = (ll)(s - 1); i >= (ll)(e); i--)
#define all(r) r.begin(), r.end()
#define rall(r) r.rbegin(), r.rend()

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;

template <typename T, typename U>
bool chmax(T& a, const U& b) {
    if (a >= b) return false;
    a = b;
    return true;
}
template <typename T, typename U>
bool chmin(T& a, const U& b) {
    if (a <= b) return false;
    a = b;
    return true;
}

void yes_no(bool f, string yes = "Yes", string no = "No") {
    cout << (f ? yes : no) << "\n";
}

void solve() {
    int n;
    cin >> n;
    vi a(n);
    rep(i, n) cin >> a[i];
    const int m = 3;
    vi cnt(m);
    rep(i, n) {
        if (a[i] < m) cnt[a[i]]++;
    }
    using D = long double;
    const int ma = 110;
    vector dp(ma, vector(ma, vector<D>(ma, (D)0)));
    vector used(ma, vector(ma, vi(ma, 0)));
    auto dfs = [&](auto self, int a, int b, int c) -> D {
        if (used[a][b][c]) return dp[a][b][c];
        used[a][b][c] = 1;
        int sum = a + b + c;
        if (sum == 0) return dp[a][b][c] = 0;
        D res = n;
        if (a > 0) res += self(self, a - 1, b + 1, c) * a;
        if (b > 0) res += self(self, a, b - 1, c + 1) * b;
        if (c > 0) res += self(self, a, b, c - 1) * c;
        res /= sum;
        return dp[a][b][c] = res;
    };
    D ans = dfs(dfs, cnt[0], cnt[1], cnt[2]);
    cout << fixed << setprecision(15);
    cout << ans << "\n";
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int t = 1;
    // cin >> t;
    rep(ti, t) solve();
    return 0;
}