import Control.Monad
import Data.Maybe
import qualified Data.ByteString.Char8 as BS
import Data.Array.Unboxed
import Data.Bits
import Data.List
import qualified  Data.ByteString as B


bsToInt :: BS.ByteString -> Int
bsToInt = fst . fromJust . BS.readInt

main :: IO ()
main = do 
    r <- bsToInt <$> BS.getLine  :: IO Int
    --rgbs <- BS.getLine :: IO BS.ByteString
    print $ 2 * pi * (fromIntegral r)
