import Data.Ratio
import Text.Printf

dp = [solve i | i <- [0..]]

solve 0 = 4 % 1
solve 1 = 3 % 1
solve k = (19 * (dp !! (k-1)) - 12 * (dp !! (k-2))) / 4

printer r = printf "%.15f" (fromRational r :: Double)

main = do
	n <- fmap read getLine
	printer . solve $ n