#pragma region Macros

#include <bits/stdc++.h>
using namespace std;
//#include <atcoder/all>
//using namespace atcoder;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pli = pair<ll, int>;
using pdd = pair<ld, ld>;
const ll LINF=0x1fffffffffffffff;
const ll MINF=0x7fffffffffff;
const int INF=0x3fffffff;
const ld DINF=numeric_limits<ld>::infinity();
const int MOD=1000000007;
const int MODD=998244353;
const ld EPS=1e-9;
const ld PI=3.1415926535897932;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1};
const int dy[] = {1, 0, -1, 0, 1, 1, -1, -1};
#define vec vector
#define elif else if
#define endl '\n'
#define setpre cout << fixed << setprecision(15);
#define overload5(a,b,c,d,e,name,...) name
#define overload4(a,b,c,d,name,...) name
#define overload3(a,b,c,name,...) name
#define rep1(n) for(ll i=0;i<(ll)(n);i++)
#define rep2(i,n) for(ll i=0;i<(ll)(n);i++)
#define rep3(i,a,b) for(ll i=(ll)(a);i<(ll)(b);i++)
#define rep4(i,a,b,c) for(ll i=(ll)(a);i<(ll)(b);i+=(ll)(c))
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep1(n) for(ll i=(ll)(n);i--;)
#define rrep2(i,n) for(ll i=(ll)(n);i--;)
#define rrep3(i,a,b) for(ll i=(ll)(b);i-->(ll)(a);)
#define rrep4(i,a,b,c) for(ll i=(ll)(a)+((ll)(b)-(ll)(a)-1)/(ll)(c)*(ll)(c);i>=(ll)(a);i-=(ll)(c);)
#define rrep(...) overload4(__VA_ARGS__,rrep4,rrep3,rrep2,rrep1)(__VA_ARGS__)
#define each1(a,v) for(auto &&a:v)
#define each2(a,b,v) for(auto &&[a,b]:v)
#define each3(a,b,c,v) for(auto &&[a,b,c]:v)
#define each4(a,b,c,d,v) for(auto &&[a,b,c,d]:v)
#define each(...) overload5(__VA_ARGS__,each4,each3,each2,each1)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define Sz(x) (x).size()
#define Sort(x) sort(all(x))
#define rSort(x) sort(rall(x))
#define Rev(x) reverse(all(x))
template<class T> auto min(const T& a){ return *min_element(all(a));}
template<class T> auto max(const T& a){ return *max_element(all(a));}
template<class T> bool chmin(T& a, const T& b){if(a>b){a=b;return 1;}return 0;}
template<class T> bool chmax(T& a, const T& b){if(a<b){a=b;return 1;}return 0;}
template<class T, class U> bool chmin(T& a, const U& b){if(a>b){a=b;return 1;}return 0;}
template<class T, class U> bool chmax(T& a, const U& b){if(a<b){a=b;return 1;}return 0;}
template<class T> using PQ = priority_queue<T, vec<T>>;
template<class T> using PQG = priority_queue<T, vec<T>, greater<T>>;
ll intpow(ll m, ll n){ll res=1;while(n>0){if(n&1)res=res*m;m=m*m;n>>=1;}return res;}
ll modpow(ll m, ll n){ll res=1;while(n>0){if(n&1)res=res*m%MOD;m=m*m%MOD;n>>=1;}return res;}

#pragma endregion Macros



int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    int a3 = 0, a2 = 0, a1 = 0;
    rep(n) {
        int a;
        cin >> a;
        if (a == 0) a3++;
        elif (a == 1) a2++;
        elif (a == 2) a1++;
    }

    int const m = 110;
    vec<vec<vec<ld>>> dp(m, vec<vec<ld>>(m, vec<ld>(m, -1)));
    dp[0][0][0] = 0;
    auto f = [&](auto f, int a3, int a2, int a1)->ld {
        if (dp[a3][a2][a1] >= 0) return dp[a3][a2][a1];

        int b = a1+a2+a3;
        ld res = (ld)n/b;
        if (a3 > 0) res += f(f, a3-1, a2+1, a1) * (ld)a3/b;
        if (a2 > 0) res += f(f, a3, a2-1, a1+1) * (ld)a2/b;
        if (a1 > 0) res += f(f, a3, a2, a1-1) * (ld)a1/b;
        dp[a3][a2][a1] = res;
        return res;
    };

    setpre;
    cout << f(f, a3, a2, a1) << endl;

    return 0;
}