#include <bits/stdc++.h>

#define mp       make_pair
#define mt	 make_tuple
#define pb       push_back
#define rep(i,n) for(int i=0;i<(n);i++)

using namespace std;

typedef    long long          ll;
typedef    unsigned long long ull;
typedef    pair<int,int>      pii;
typedef    pair<long,long>    pll;

const int INF=1<<29;
const double EPS=1e-9;
const int MOD = 100000007;

const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};
int N;
double dp[110][110][110];

double memo(int two, int one, int zero){
  if (dp[two][one][zero] >= 0){
    return dp[two][one][zero];
  }
  if (two == 0 && one == 0 && zero == 0){
    return 0.0;
  }
  double res = N * 1.0 / (two + one + zero);
  if (two > 0){
    res += memo(two - 1, one, zero) * two / (two + one + zero);
  }
  if (one > 0){
    res += memo(two + 1, one -1, zero) * one / (two + one + zero);
  }
  if (zero > 0){
    res += memo(two, one + 1, zero - 1) * zero / (two + one + zero);
  }
  return dp[two][one][zero] = res;
}

int main(){
  cin >> N;
  int two,one,zero;
  two = one = zero = 0;
  for (int i = 0; i < N; i++){
    int x;
    cin >> x;
    if (x == 0){
      zero++;
    }else if(x == 1){
      one++;
    }else if(x == 2){
      two++;
    }
  }
  memset(dp, -1, sizeof(dp));
  printf("%.20lf\n", memo(two, one, zero));
}