#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define loop(i,a,b) for(ll i=(a);i<ll(b);i++)
#define rep(i,b) loop(i,0,b)

int n;

double dp[111][111][111];
double rec(int a, int b, int c){
    double & res = dp[a][b][c];
    if(res != -1) return res;
    int s = a+b+c;
    if(s == 0) return res = 0;
    res = 0;
    if(a) res += rec(a-1,b+1,c) * a/s;
    if(b) res += rec(a,b-1,c+1) * b/s;
    if(c) res += rec(a,b,c-1) * c/s;
    res += 1.*n/s;
    return res;
}

int main(){
    cin >> n;
    int a = 0, b = 0, c = 0;
    rep(i,n){
        int x;
        cin >> x;
        if(x==0) a++;
        if(x==1) b++;
        if(x==2) c++;
    }
    rep(i,111)rep(j,111)rep(k,111) dp[i][j][k] = -1;
    printf("%.10lf\n",rec(a,b,c));
}