#include <bits/stdc++.h>

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = std::int64_t;
using P = std::tuple<int,int>;

int N;
int A[100];
double dp[101][101][101];

double rec(int n0, int n1, int n2){
    if(n0 + n1 + n2 == 0){
        return 0;
    }

    double p0 = 1.0 * n0 / N,
        p1 = 1.0 * n1 / N,
        p2 = 1.0 * n2 / N,
        p = p0 + p1 + p2;
    
    double e = 1;

    if(n0 > 0){
        e += p0 * rec(n0 - 1, n1 + 1, n2);
    }

    if(n1 > 0){
        e += p1 * rec(n0, n1 - 1, n2 + 1);
    }

    if(n2 > 0){
        e += p2 * rec(n0, n1, n2 - 1);
    }

    e /= p;

    return dp[n0][n1][n2] = e;
}

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    std::cin >> N;

    for(int i=0;i<N;++i){
        std::cin >> A[i];
    }

    std::fill(&dp[0][0][0], &dp[0][0][0] + 101 * 101 * 101, -1);

    double e = rec(std::count(A, A + N, 0),
                   std::count(A, A + N, 1),
                   std::count(A, A + N, 2));
    printf("%.10f\n", e);
}