n=gets.to_i
pi=gets.split.map(&:to_f)

arr = (0..n).map{|i|[0]*(i+1)}
arr[0][0]=1

pi.each.with_index{|pii,i|
  arii = arr[i+1]
  rpii = 1-pii
  arr[i].each.with_index{|ai,j|
    arii[j] += ai * rpii
    arii[j+1] += ai * pii
  }
}
p arr[n][-n/2..-1].inject(:+)