#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
using namespace std;
double memo[101][101][101];
int n;

double func(int a, int b, int c)
{
    if (a==0 and b==0 and c==0) return 0;

    auto& res=memo[a][b][c];
    // if (!isnan(res)) return res;
    if (res==res) return res;

    int s=a+b+c;
    res=(double)n/s;
    if (a) res+=func(a-1, b+1, c)*a/s;
    if (b) res+=func(a, b-1, c+1)*b/s;
    if (c) res+=func(a, b, c-1)*c/s;
    return res;
}

int main()
{
    cin>>n;
    int m[3]={};
    vector<int> a(n);
    for(int& e: a) {
        cin>>e;
        if (e<3) m[e]++;
    }
    memset(memo, -1, sizeof(memo));
    printf("%.9f\n", func(m[0], m[1], m[2]));
}