import sys
sys.setrecursionlimit(10**6)

n = int(input())
A = list(map(int, input().split()))
C = [0, 0, 0, 0]
for i in range(n):
    a = min(A[i], 3)
    C[a] += 1

memo = dict()
memo[(0, 0, 0, n)] = 0.0
def E(X):
    if X in memo:
        return memo[X]
    Z = list(X)
    res = 0.0
    for i in range(3):
        if Z[i] == 0:
            continue
        Z[i] -= 1
        Z[i + 1] += 1
        res += (Z[i] + 1) * (E(tuple(Z)) + 1)
        Z[i] += 1
        Z[i + 1] -= 1
    res += Z[3]
    res /= n - Z[3]
    memo[X] = res
    return res

print(E(tuple(C)))