#include <bits/stdc++.h>
#include <stdlib.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vec;
typedef vector<vec> mat;
typedef pair<ll,ll> P;
typedef priority_queue<P,vector<P>,greater<P>> P_queue;

const ll MOD=998244353;
const ll mod=1000000007;
const ll INF=1e15;
vec dx={1,0,-1,0};
vec dy={0,1,0,-1};

#define REP(i,a,b) for(int i=(int)a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define mp make_pair
#define ALL(a) a.begin(),a.end()
#define SORT(a) sort(ALL(a))
#define U_ERASE(V) V.erase(unique(ALL(V)), V.end());
#define ADD(a,b) a=(a+b)%mod


double dp[101][101][101];
bool arrived[101][101][101];
ll N;
ll A,B,C;

ll Count=0;
double Solve(ll a, ll b, ll c){
    if(arrived[a][b][c]) return dp[a][b][c];
    arrived[a][b][c]=true;

    if(c>=N) return dp[a][b][c]=0;
    //Count++;
    //if(Count>1e7) return -1;
    //cout<<Count<<':'<<a<<' '<<b<<' '<<c<<endl;
    double ret=1;
    if(a<N) ret+=((N-a)*Solve(a+1,b,c))/N;
    if(a>b && b<N) ret+=((a-b)*Solve(a,b+1,c))/N;
    if(b>c && c<N) ret+=((b-c)*Solve(a,b,c+1))/N;
    return dp[a][b][c]=(N*ret)/(N-c);

}
int main(){
    cin>>N;
    rep(i,N){
        ll x; cin>>x;
        if(x>=3) C++;
        if(x>=2) B++;
        if(x>=1) A++;
    }
    rep(i,101) rep(j,101) rep(k,101) dp[i][j][k]=-5;
    cout<<setprecision(9)<<Solve(A,B,C)<<endl;

}