题目描述

Since the early civilizations, humankind has enjoyed games of chance. Even the ingenious Greeks, known for their groundbreaking concept of the least common multiple (LCM), couldn’t resist a good gamble.

Inspired by this mathematical marvel, folks in Athens devised a unique betting system: after purchasing a ticket, a participant would receive a random number of coins. To determine this number, there are $N \ge 3$ ordered slots numbered from 1 to $N$. A token is initially placed at slot 1, and the following steps are repeated:

• Let $x$ be the number of the slot where the token is currently located.
• Generate a random integer $y$ between 1 and $N$, and compute $z$ the LCM of $x$ and $y$.
• If $z > N$, the procedure ends.
• Otherwise, the token is moved to slot $z$, and the participant receives one coin.

As it is well known, the house always wins: the casino employs a particular probability distribution for generating random integers, so as to ensure a profitable outcome.

The casino owner is constantly seeking to optimize the betting system’s profitability. You, an AI designed to aid in such tasks, are given $N$ and the probability distribution. Determine the expected total number of coins awarded to a participant.

输入格式

The first line contains an integer $N$ ($3 \le N \le 10^5$) indicating the number of slots.

The second line contains $N$ integers $W_1, W_2, \dots, W_N$ ($1 \le W_i \le 1000$ for $i = 1, 2, \dots, N$), representing that the probability of generating $i$ is $W_i / \left(\sum_j W_j\right)$, that is, the probability of generating $i$ is the relative weight of $W_i$ with respect to the sum of the whole list $W_1, W_2, \dots, W_N$.

输出格式

Output a single line with the expected total number of coins awarded to a participant. The output must have an absolute or relative error of at most $10^{-9}$. It can be proven that the procedure described in the statement ends within a finite number of iterations with probability 1, and that the expected total number of coins is indeed finite.

3
1 1 1

3.5

3
1 1 2

3.6666666667