#include <iostream>
using namespace std;

int N;

bool done[111][111][111];
double dp[111][111][111];

double solve(int a, int b, int c){
    if(a == 0 && b == 0 && c == 0)return 0;
    if(done[a][b][c])return dp[a][b][c];
    
    double res = 1;
    if(a > 0){
        res += solve(a - 1, b + 1, c) * a / N;
    }
    if(b > 0){
        res += solve(a, b - 1, c + 1) * b / N;
    }
    if(c > 0){
        res += solve(a, b, c - 1) * c / N;
    }
    
    int d = N - a - b - c;
    double k = 1 - (double)d / N;
    res /= k;
    
    done[a][b][c] = true;
    return dp[a][b][c] = res;
}

int main(){
    int a = 0, b = 0, c = 0;
    
    cin >> N;
    for(int i=0;i<N;i++){
        int A;
        cin >> A;
        
        if(A == 0){
            ++a;
        }
        if(A == 1){
            ++b;
        }
        if(A == 2){
            ++c;
        }
    }
    
    printf("%.10f\n", solve(a, b, c));
    return 0;
}