#include <iostream>
#include <cstdio>

using namespace std;

const double EPS = 1e-9;

int arr[4];
double dp[305][305][305];
int n;

double solve(int a, int b, int c)
{
    if(a==0&&b==0&&c==0) return 0;
    if(dp[a][b][c]>0) return dp[a][b][c];
    else {
        double ans = n*1.0/(a+b+c);
        if(a) ans += 1.0*a/(a+b+c)*solve(a-1, b, c);
        if(b) ans += 1.0*b/(a+b+c)*solve(a+1, b-1, c);
        if(c) ans += 1.0*c/(a+b+c)*solve(a, b+1, c-1);
        return dp[a][b][c] = ans;
    }
}

int main()
{
    scanf("%d", &n);
    int t;
    for(int i=0;i<n;++i) {
        scanf("%d", &t);
        arr[t]++;
    }
    printf("%.9lf\n", solve(arr[1], arr[2], arr[3]));
    return 0;
}