import qualified Data.Text as T
import qualified Data.Text.IO as T
import qualified Data.Text.Read as T

main :: IO ()
main = do
  r <- unsafeTextToDouble <$> T.getLine :: IO Double
  print $ 2 * pi * r

unsafeTextToDouble :: T.Text -> Double
unsafeTextToDouble s = case T.double s of
  Right (x, _) -> x
