#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

int N, A[100], cntNeed[4], vis[101][101][101];
double loop[101], memo[101][101][101];

double calc(int one, int two, int three) {
    double &res = memo[one][two][three];
    if(vis[one][two][three]++)return res;
    int need = one + two + three;
    if(need == 0)return res=0;
    if(one > 0)res += calc(one - 1, two, three) * one;
    if(two > 0)res += calc(one + 1, two - 1, three)*two;
    if(three > 0)res += calc(one, two + 1, three - 1)*three;
    res /= need;
    res += 1 + loop[need];
    return res;
}

int main(){
    cin >> N;
    rep(i, N) {
        cin >> A[i];
        cntNeed[3 - min(A[i], 3)]++;
    }
    FOR(i, 1, N) {
        double pw = 1, mul = (double)(N - i) / N, hit = 1-mul;
        rep(j, 10000) {
            loop[i] += j*pw;
            pw *= mul;
        }
        loop[i] *= hit;
    }
    double ans = calc(cntNeed[1], cntNeed[2], cntNeed[3]);
    cout << setprecision(20) << ans << endl;
}