import Text.Printf

main :: IO ()
main = do --interact $ show . solve . map read . words
    d <- solve . map read . words <$> getContents
    printf "%.5f" d

solve :: [Int] -> Double
solve [cal, rin, rout] = (fromIntegral cal) * t
    where
        r = (fromIntegral rout - fromIntegral rin) / 2
        rhalf = (fromIntegral rout + fromIntegral rin) / 2
        s = pi * r * r
        t = s * (2 * pi * rhalf)