Problem:
Score : 100 points

Problem Statement

Let N be a positive odd number.

There are N coins, numbered 1, 2, \ldots, N.
For each i (1 \leq i \leq N), when Coin i is tossed, it comes up heads with probability p_i and tails with probability 1 - p_i.

Taro has tossed all the N coins.
Find the probability of having more heads than tails.

Constraints

N is an odd number.

1 \leq N \leq 2999

p_i is a real number and has two decimal places.

0 < p_i < 1

Input

Input is given from Standard Input in the following format:

N
p_1 p_2 \ldots p_N

Output

Print the probability of having more heads than tails.
The output is considered correct when the absolute error is not greater than 10^{-9}.

Sample Input 1

3
0.30 0.60 0.80

Sample Output 1

0.612

The probability of each case where we have more heads than tails is as follows:

The probability of having (Coin 1, Coin 2, Coin 3) = (Head, Head, Head) is 0.3 × 0.6 × 0.8 = 0.144;

The probability of having (Coin 1, Coin 2, Coin 3) = (Tail, Head, Head) is 0.7 × 0.6 × 0.8 = 0.336;

The probability of having (Coin 1, Coin 2, Coin 3) = (Head, Tail, Head) is 0.3 × 0.4 × 0.8 = 0.096;

The probability of having (Coin 1, Coin 2, Coin 3) = (Head, Head, Tail) is 0.3 × 0.6 × 0.2 = 0.036.

Thus, the probability of having more heads than tails is 0.144 + 0.336 + 0.096 + 0.036 = 0.612.

Sample Input 2

1
0.50

Sample Output 2

0.5

Outputs such as 0.500, 0.500000001 and 0.499999999 are also considered correct.

Sample Input 3

5
0.42 0.01 0.42 0.99 0.42

Sample Output 3

0.3821815872

Solution in PyPy3 (7.3.0) (judge verdict: Accepted)

N = int(input())
p = [-1] + list(map(float, input().split()))
dp = [[1] * (N+1) for _ in range(N+1)] #i番目のコインまででj枚表が出る確率
dp[1][0] = 1 - p[1]
dp[1][1] = p[1]
for i in range(2, N + 1):
    for j in range(i + 1):
        if j == 0: #表０枚の確率
            dp[i][j] = dp[i-1][j] * (1 - p[i])
        elif j == i:
            dp[i][j] = dp[i-1][j-1] * p[i]
        else:
            dp[i][j] = dp[i-1][j] * (1 - p[i]) + dp[i-1][j-1] * p[i]
num = N // 2 + 1
ans = 0
while num <= N:
    ans += dp[N][num]
    num += 1
print(ans)