local a, b = io.read("*n", "*n")
local sum = 0
local eps = 1.0e-7
local pi2 = math.pi * 2
local msq = math.sqrt
for x = a, b, eps do
  local y = msq(1 - x * x)
  if(0 < y) then
    sum = sum + eps / y * pi2 * y
  end
end
print(string.format("%.8f", sum))