import Control.Monad
import Data.Maybe
import qualified Data.ByteString.Char8 as BS

main = abc163a

abc163a = do
    [r] <- getIntList
    print $ 2.0 * pi * fromIntegral r

readInt = fst . fromJust . BS.readInteger
readIntList = map readInt . BS.words
getIntList = readIntList <$> BS.getLine