numericalIntegration :: Double -> Double -> Double -> (Double -> Double) -> Double
numericalIntegration d a b f
  | b < a = numericalIntegration d b a f
  | otherwise = sum [(f (a+d*i) + f (a+d*(i+1)))*d/2 | i<-[0..n-1]]
  where n = (b-a)/d

main = do
  a <- readLn :: IO Double
  b <- readLn :: IO Double

  let d = 0.00001
  
  print $ numericalIntegration d a b (\x -> abs $ 4*pi*x)