#include <iostream>
#include <iomanip>
using namespace std;

const int MAX_N = 110;

double memo[MAX_N][MAX_N][MAX_N];

int n;

double calc(int n0, int n1, int n2) {
    if (memo[n0][n1][n2] >= 0.0) return memo[n0][n1][n2];
    int sum = n0 + n1 + n2;
    if (sum == 0) {
        return memo[n0][n1][n2] = 0.0;
    }
    double res = (double)n / (double)sum;
    if (n0 > 0) res += (double)n0 / (double)sum * calc(n0 - 1, n1 + 1, n2);
    if (n1 > 0) res += (double)n1 / (double)sum * calc(n0, n1 - 1, n2 + 1);
    if (n2 > 0) res += (double)n2 / (double)sum * calc(n0, n1, n2 - 1);
    return memo[n0][n1][n2] = res;
}

int main() {
    cin >> n;

    for (int n0 = 0; n0 < MAX_N; n0++) {
        for (int n1 = 0; n1 < MAX_N; n1++) {
            for (int n2 = 0; n2 < MAX_N; n2++) {
                memo[n0][n1][n2] = -1.0;
            }
        }
    }

    int n0 = 0, n1 = 0, n2 = 0;
    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        if (a == 0) n0++;
        else if (a == 1) n1++;
        else if (a == 2) n2++;
    }
    cout << fixed << setprecision(10) << calc(n0, n1, n2) << endl;
    return 0;
}