N = int(input())
A = list(map(int,input().split()))
B = [0,0,0,0]
for a in A:
    if a >= 3:
        B[3] += 1
    else:
        B[a] += 1
n0 = B[0]; n1 = B[1]; n2 = B[2]
nS = n0+n1+n2+1
dp = [[[-1]*nS for _ in range(nS)] for _ in range(nS)]
dp[0][0][0] = 0

def solve(n0,n1,n2):
    ns = n0 + n1 + n2
    if ns > N or n0 < 0 or n1 < 0 or n2 < 0:
        return 0
    elif dp[n0][n1][n2] != -1:
        return dp[n0][n1][n2]
    else:
        v = (N + n0*solve(n0-1,n1+1,n2)+n1*solve(n0,n1-1,n2+1)+n2*solve(n0,n1,n2-1))/ns
        dp[n0][n1][n2] = v
        return v
        
print(solve(n0,n1,n2))