#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
double dp[110][110][110];
int main()
{
    int n;
    cin >> n;
    vector<int> v(4, 0);
    rep(i, n)
    {
        int a;
        cin >> a;
        if(a > 3) a = 3;
        v[a]++;
    }
    memset(dp, 0, sizeof(dp));
    function<double(int, int, int)> rec = [&](int i, int j, int k)
    {
        if(dp[i][j][k] > 0) return dp[i][j][k];
        if(k == n) return 0.0;
        double res = 0;
        if(i > 0) res += i * rec(i-1, j+1, k);
        if(j > 0) res += j * rec(i, j-1, k+1);
        if(n-i-j-k > 0) res += (n-i-j-k) * rec(i+1, j, k);
        res += n;
        res /= n-k;
        return dp[i][j][k] = res;
    };
    cout << fixed << setprecision(10) << rec(v[1], v[2], v[3]) << endl;
}