#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
ll n;
double memo[110][110][110];
double dfs(ll a,ll b,ll c){
    if(a<0||b<0||c<0)return 0;
    if(memo[a][b][c]!=-1)return memo[a][b][c];
    double ret=1;
    ret+=dfs(a-1,b+1,c)*a/n;
    ret+=dfs(a,b-1,c+1)*b/n;
    ret+=dfs(a,b,c-1)*c/n;
    ret*=n;ret/=(a+b+c);
    memo[a][b][c]=ret;
    return ret;
}
int main(){
    cin >> n;
    rep(i,110)rep(j,110)rep(k,110)memo[i][j][k]=-1;
    memo[0][0][0]=0;
    ll a=0,b=0,c=0;
    rep(i,n){
        ll x;cin >> x;
        if(x==0)a++;
        if(x==1)b++;
        if(x==2)c++;
    }
    CST(10);
    cout << dfs(a,b,c) <<endl;
}