local mmi, mma = math.min, math.max
local x, y = io.read("*n", "*n")
local z, w = io.read("*n", "*n")

local amin = mma(z, y - w)
local amax = 1000000007
for irep = 1, 2000 do
  local a = (amin + amax) / 2
  local v = (a * a - z * z) * ((a + w) * (a + w) - y * y)
  if v <= x * x then
    amin = a
  else
    amax = a
  end
end
local a = amin
local height = math.sqrt((a + w) * (a + w) - y * y)
local width = math.sqrt(a * a - z * z)
print(x - y * height / 2 - z * width / 2 - (width - y) * (height - z) / 2)