#include <bits/stdc++.h>
using namespace std;

// #define int long long
#define rep(i, n) for (long long i = (long long)(0); i < (long long)(n); ++i)
#define reps(i, n) for (long long i = (long long)(1); i <= (long long)(n); ++i)
#define rrep(i, n) for (long long i = ((long long)(n)-1); i >= 0; i--)
#define rreps(i, n) for (long long i = ((long long)(n)); i > 0; i--)
#define irep(i, m, n) for (long long i = (long long)(m); i < (long long)(n); ++i)
#define ireps(i, m, n) for (long long i = (long long)(m); i <= (long long)(n); ++i)
#define irreps(i, m, n) for (long long i = ((long long)(n)-1); i > (long long)(m); ++i)
#define SORT(v, n) sort(v, v + n);
#define REVERSE(v, n) reverse(v, v+n);
#define vsort(v) sort(v.begin(), v.end());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cinline(n) getline(cin,n);
#define replace_all(s, b, a) replace(s.begin(),s.end(), b, a);
#define PI (acos(-1))
#define FILL(v, n, x) fill(v, v + n, x);
#define sz(x) (long long)(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll,ll,ll>>;
using vb = vector<bool>;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

const ll INF = 1e9+10;
const ll MOD = 1e9+7;
const ll LINF = 1e18;

double dp[105][105][105];
ll n;
double rec(ll x, ll y, ll z){
  if(z>=n) return 0;
  if(dp[x][y][z]) return dp[x][y][z];
  
  double res=1;
  ll zero=n-(x+y+z);
  if(zero>0) res+=rec(x+1,y,z)*((double)zero/n);
  if(x) res+=rec(x-1,y+1,z)*((double)x/n);
  if(y) res+=rec(x,y-1,z+1)*((double)y/n);
  res=res*n*1.0/(n-z);
  return dp[x][y][z]=res;
}

signed main()
{
  cin.tie( 0 ); ios::sync_with_stdio( false );
  
  cin>>n;
  ll x,y,z; x=y=z=0;
  rep(i,n) {
    ll a; cin>>a;
    if(a==1) x++;
    if(a==2) y++;
    if(a>=3) z++;
  }
  
  double ans=rec(x,y,z);
  printf("%.15lf\n", ans);
  
}