# -*- coding: utf-8 -*-
# Created by Ross on 19-4-12

n = int(input())
if n < 1:
    exit()

neg = 1  # 表示前面两人都抽不中的概率
c = 0

ps = []
for i in range(n):
    p = float(input())
    ps.append(p)

i = 0
while i < 100:
    if i % 2 == 0:  # 轮到小明
        p = ps[i % n]
        c += neg * p
        neg *= (1 - p)
    else:  # 轮到小华
        p = ps[i % n]
        neg *= 1 - p
    i += 1
print('%.4f' % c)