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//#pragma optimization_level 3
 
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
 
#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <chrono>
#include <ctime>
#include <queue>
#include <math.h>
#include <deque>
#include <stack>
#include <iomanip>
#include <assert.h>
#include <stdio.h>
#include <cstring>
#include <random>
#include <limits.h>
 
using namespace std;
 
#define int long long
#define ll long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define pld pair<ld, ld>
#define vi vector <int>
#define vpi vector <pii>
#define vld vector <ld>
#define try tr
#define left left11
#define right right11
#define lb lower_bound
#define ub upper_bound
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define pf push_front
#define sqr(a) ((a) * (a))
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
 
#define x first
#define y second
 
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
 
clock_t START;
 
inline ld TIME() {
    return (ld) (clock() - START) / CLOCKS_PER_SEC;
}

void SHOW() {
    cout << TIME() << " SECONDS FROM START\n";
}
 
ld TL = 2.0; 
int calls = 0;
 
inline bool IS() {
    if(++calls == 1000) {
        calls = 0;
        if(TL - TIME() < 0.1)
            return true;
    }
    return false;
}
 
template<typename T1, typename T2>
inline void amin(T1 &a, T2 b) { if (a > b) a = b; }
 
template<typename T1, typename T2>
inline void amax(T1 &a, T2 b) { if (a < b) a = b; }
 
template<typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> &p) {
    os << p.first << ' ' << p.second << endl;
    return os;
}
 
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
    is >> p.first >> p.second;
    return is;
}
 
template<typename T>
istream &operator>>(istream &is, vector<T> &v) {
    for (auto &u : v) is >> u;
    return is;
}
 
template<typename T>
ostream &operator<<(ostream &os, vector<T> &v) {
    for (auto &u : v) os << u << ' ';
    os << endl;
    return os;
}
 
int hash_mod[4] = {1000000007, 998244353, 1000000009, 999999937}, mod = hash_mod[rnd() % 4];
int hash_pows[4] = {179, 239, 1007, 2003}, P = hash_pows[rnd() % 4];
 
int binpow(int x, int p) {
    int res = 1;
    while(p) {
        if(p % 2)
            res = (res * x) % mod;
        x = (x * x) % mod;
        p /= 2;
    }
    return res;
}
 
int sum(int x, int y) {
    x += y;
    if(x >= mod)
        x -= mod;
    return x;
}
 
int mult(int x, int y) {
    return (x * y) % mod;
}
 
const int N = 1e5 + 7, NS = 3e3 + 7, lg = 20, sq = 550, inf = 1e9 + 7, SM = 1e2 + 7;
const ld eps = 1e-9;
ld d[301][301][301], n;
ld calc(int x, int y, int z) {
    if(d[x][y][z])
        return d[x][y][z];
    if(max({x, y, z}) == 0)
        return d[x][y][z];
    ld tot = x + y + z;
    if(x)
        d[x][y][z] += (calc(x - 1, y, z) * (ld) x) / tot;
    if(y)
        d[x][y][z] += (calc(x + 1, y - 1, z) * (ld) y) / tot;
    if(z)
        d[x][y][z] += (calc(x, y + 1, z - 1) * (ld) z) / tot;
    d[x][y][z] += n / tot;
    return d[x][y][z];
}
void solve() {
    cout << fixed << setprecision(10);
    cin >> n;
    int x = 0, y = 0, z = 0;
    for (int i = 0; i < n; i++) {
        int k;
        cin >> k;
        if(k == 1)
            x++;
        if(k == 2)
            y++;
        if(k == 3)
            z++;
    }
    cout << calc(x, y, z);
}
 
signed main() {
    fast
    solve();
}

Measured memory_bytes: 212908