local read = setmetatable({}, {__index = function(t, k) local a = {} for i=1,#k do table.insert(a, '*'..string.sub(k, i, i)) end local r = io.read local u = table.unpack or unpack return function() return r(u(a)) end end})
read.N = function(N) local t={} for i=1,N do t[i]=read.n() end return t end
string.totable = function(s) local t={} local u=string.sub for i=1,#s do t[i] = u(s, i, i) end return t end
string.split = function(s) local t={} for w in string.gmatch(s, "[^%s]+") do table.insert(t, w) end return (table.unpack or unpack)(t) end
local function array(dimension, default_val) local n=dimension local m={}if default_val~=nil then m[1]={__index=function()return default_val end}end for i=2,n do m[i]={__index=function(p, k)local c=setmetatable({},m[i-1])rawset(p,k,c)return c end}end return setmetatable({},m[n])end
----
local N = read.n()
local ph = read.N(N)
local pt = {}
for i=1,N do pt[i] = 1 - ph[i] end

local tbl = array(2, 0)
tbl[0][0] = 1.

for i=1,N do
    tbl[i][0] = tbl[i-1][0] * pt[i]
    for j=1,i do
        tbl[i][j] = tbl[i-1][j] * pt[i] + tbl[i-1][j-1] * ph[i]
    end
end
local sum = 0
for c=math.ceil(N/2),N do
    sum = sum + tbl[N][c]
end

print(sum)