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

typedef long long int ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pair<int, int> > vii;
#define rrep(i, m, n) for(int (i)=(m); (i)<(n);  (i)++)
#define erep(i, m, n) for(int (i)=(m); (i)<=(n); (i)++)
#define  rep(i, n)    for(int (i)=0; (i)<(n);  (i)++)
#define  rev(i, n)    for(int (i)=(n)-1; (i)>=0; (i)--)
#define vrep(i, c)    for(__typeof((c).begin())i=(c).begin(); i!=(c).end(); i++)
#define  ALL(v)       (v).begin(), (v).end()
#define mp            make_pair
#define pb            push_back
template<class T1, class T2> inline void minup(T1& m, T2 x){ if(m>x) m=static_cast<T2>(x); }
template<class T1, class T2> inline void maxup(T1& m, T2 x){ if(m<x) m=static_cast<T2>(x); }

#define INF 1000000000.0
#define MOD 1000000007
#define EPS 1E-12

const int MAX_N = 100;
int N;
int A[MAX_N];
int card[3];
double dp[MAX_N+1][MAX_N+1][MAX_N+1];

double solve(int x, int y, int z)
{
  if(dp[x][y][z] >= 0) return dp[x][y][z];
  if(x + y + z <= 0) return dp[x][y][z] = 0.0;

  double res = (double)N / (x + y + z);

  if(x > 0) res += solve(x - 1, y + 1, z    ) * x / (x + y + z);
  if(y > 0) res += solve(x,     y - 1, z + 1) * y / (x + y + z);
  if(z > 0) res += solve(x,     y,     z - 1) * z / (x + y + z);

  return dp[x][y][z] = res;
}

int main()
{
  cin >> N;
  rep(i, N) cin >> A[i];
  rep(i, N) rep(j, 3) if(A[i] == j) card[j] += 1;

  erep(i, 0, N) erep(j, 0, N) erep(k, 0, N) dp[i][j][k] = -INF;

  printf("%.10f\n",  solve(card[0], card[1], card[2]));

  return 0;
}