volTorus :: [Double] -> Double
volTorus (rin : rout : _) = 2 * pi * pi * rmin * rmin * rmax
    where
        rmin = rout / 2 - rin / 2
        rmax = rout / 2 + rin / 2

main :: IO ()
main = do
    c <- readLn
    print . (*) c . volTorus . map read . words =<< getLine