#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.14159265359;
ll mod=998244353;
ll inf=99999999999999999;
vector<vector<vector<ld>>>dp;
ll n;
ll one=0,two=0,thr=0;
ld saiki(ll a,ll b,ll c){
    if (a+b+c>n||a<0||b<0)
    {
        return 0;
    }
    if (dp[a][b][c]!=-1)
    {
        return dp[a][b][c];
    }
    ld pl=(ld)n/(ld)(n-c);
    ld y=0;
    y+=(saiki(a+1,b,c)+pl)*((ld)(n-a-b-c)/(ld)(n-c));
    y+=(saiki(a-1,b+1,c)+pl)*((ld)(a)/(ld)(n-c));
    y+=(saiki(a,b-1,c+1)+pl)*((ld)(b)/(ld)(n-c));
    dp[a][b][c]=y;
    return dp[a][b][c];
}
int main(){
    cin >> n;
    vector<ll>a(n);
    for (ll i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    ll one=0,two=0,thr=0;
    for (ll i = 0; i < n; i++)
    {
        if (a[i]==1)
        {
            one+=1;
        }
        if (a[i]==2)
        {
            two+=1;
        }
        if (a[i]>=3)
        {
            thr+=1;
        }
    }
    dp.assign(n+1,vector<vector<ld>>(n+1,vector<ld>(n+1,-1)));
    dp[0][0][n]=0;
    cout << setprecision(20) << saiki(one,two,thr) << endl;
}