import sys
sys.setrecursionlimit(10**7)
N = int(input())        
A = list(map(int,input().split()))
dp = [[[0 for _ in range(N+1)] for _ in range(N+1)] for _ in range(N+1)]
memo = [[[False for _ in range(N+1)] for _ in range(101)] for _ in range(101)]
memo[N][N][N] = True
def DP(x,y,z):
    if x == N and y == N and z == N:
        return 
    if x >= y >= z:
        pass
    else:
        return
    if x + 1 <= N:
        if memo[x+1][y][z]:
            dp[x][y][z] += (N-x) * (dp[x+1][y][z])
        else:
            DP(x+1,y,z)
            dp[x][y][z] += (N-x) * (dp[x+1][y][z]) 
    if y + 1 <= N:
        if memo[x][y+1][z]: 
            dp[x][y][z] += (x-y) * (dp[x][y+1][z]) 
        else:
            DP(x,y+1,z)
            dp[x][y][z] += (x-y) * (dp[x][y+1][z]) 
    if z + 1 <= N:
        if memo[x][y][z+1]:
            dp[x][y][z] += (y-z) * (dp[x][y][z+1]) 
        else:
            DP(x,y,z+1)
            dp[x][y][z] += (y-z) * (dp[x][y][z+1]) 
    dp[x][y][z] += N
    dp[x][y][z] /=  N -z
    memo[x][y][z] = True
x = 0
y = 0
z = 0
for i in range(N):
    if A[i] >= 1:
        x += 1
    if A[i] >= 2:
        y += 1
    if A[i] >= 3:
        z += 1
DP(x,y,z)
print(dp[x][y][z])