#include <bits/stdc++.h>

using namespace std;

#define INF 100000000
#define YJ 1145141919
#define INF_INT_MAX 2147483647
#define INF_LL 9223372036854775
#define INF_LL_MAX 9223372036854775807
#define EPS 1e-10
#define MOD 1000000007
#define MOD9 998244353
#define Pi acos(-1)
#define LL long long
#define ULL unsigned long long
#define LD long double

#define int long long

using II = pair<int, int>;

int gcd(int a, int b) { return b != 0 ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a * b / gcd(a, b); }
int extgcd(int a, int b, int &x, int &y) { int g = a; x = 1; y = 0; if (b != 0) g = extgcd(b, a % b, y, x), y -= (a / b) * x; return g; }

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(a)  begin((a)), end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())

const int MAX_N = 105;
int N;
int A[MAX_N];
int sumA[3];

double dp[MAX_N][MAX_N][MAX_N];

double solve(int i, int j, int k) {
  if(i == sumA[0] && j == sumA[1] & k == sumA[2]) {
    return 0.0;
  }

  if(i == 0 && j == 0 && k == 0) {
    return (solve(i,j,k+1) + 1) / N;
  }

  if(dp[i][j][k] >= 0.0) {
    return dp[i][j][k];
  }

  double ret = 0.0;

  if(j-1 >= 0) {
    ret += solve(i+1,j-1,k) * (i+1) + (i+1);
  }
  if(k-1 >= 0) {
    ret += solve(i,j+1,k-1) * (j+1) + (j+1);
  }
  if(i+j+k+1 <= sumA[0] + sumA[1] + sumA[2]) {
    ret += solve(i,j,k+1) * (k+1) + (k+1);
  }

  return dp[i][j][k] = ret / (i+j+k);
}

signed main()
{
  cin >> N;
  REP(n,N) {
    cin >> A[n];
    if(A[n] >= 3) {
      continue;
    } else {
      sumA[A[n]]++;
    }
  }

  REP(i,MAX_N) {
    REP(j,MAX_N) {
      REP(k,MAX_N) {
        dp[i][j][k] = -1.0;
      }
    }
  }

  printf("%.15f\n", solve(0,0,0));

  return 0;
}