import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
A=[int(i) for i in input().split()]
DP=[[[0]*(n+1) for i in range(n+1)] for j in range(n+1)]
for i in range(n+1):
  for j in range(n+1):
    for k in range(n+1):
      if i==0 and j==0 and k==0:
        continue
      elif i+j+k>n:
        continue  
      else:
        DP[i][j][k]=1
        if i>0:
          DP[i][j][k]+=i/n*DP[i-1][j+1][k]
        if j>0:
          DP[i][j][k]+=j/n*DP[i][j-1][k+1]
        if k>0:
          DP[i][j][k]+=k/n*DP[i][j][k-1]
        DP[i][j][k]*=n/(i+j+k)
C=[0]*3
for a in A:
  if a<3:
    C[a]+=1
c1,c2,c3=C
print(DP[c1][c2][c3])