import Foreign.C.Types
import Text.Printf (printf)

foreign import ccall safe "static pow" pow :: CDouble -> CDouble -> CDouble

cPow :: Double -> Double -> Double
cPow x y = realToFrac $ pow (realToFrac x) (realToFrac y)

main :: IO ()
main = do
    n <- readLn :: IO Double
    printf "%.15f\n" (4 * cPow 0.75 n)