#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>

using namespace std;

int N;
long double ans[101][101][101];

long double f(int a, int b, int c){
    if (a < 0 || b < 0 || c < 0) return 0;
    if (a == 0 && b == 0 && c == 0) return 0;
    if (ans[a][b][c] != -1) return ans[a][b][c];
    long double res;
    res = f(a-1, b, c) * a + f(a+1, b-1, c) * b + f(a, b+1, c-1) * c + (long double) N;
    res /= (long double) (a+b+c);
    return ans[a][b][c] = res;
}

int main(){

    for (int i=0; i<=100; i++) for (int j=0; j<=100; j++) for (int k=0; k<=100; k++) ans[i][j][k] = -1;

    int a=0, b=0, c=0, d;
    cin >> N;

    for (int i=0; i<N; i++){
        cin >> d;
        if (d == 0) c++;
        else if (d == 1) b++;
        else if (d == 2) a++;
    }

    cout << setprecision(18) << f(a, b, c) << endl;

    return 0;
}