def solve(y):
    y, n  = sorted(y,reverse=True), len(y) 
    num_replicas, total = 0, 1
    factorial_i_minus_1 = 1
    
    for i in range(2,n+1):
        factorial_i_minus_1 *= i-1
        is_replica = y[i-1]==y[i-2]
        num_replicas = is_replica * (is_replica + num_replicas)
        total += i * total + factorial_i_minus_1 * (i - num_replicas)
        
    return ["{0:.2f}".format(total/factorial_i_minus_1/n)]