import Control.Monad
import Data.List

getInts :: IO [Int]
getInts = liftM (map read . words) getLine

main = do
  n <- readLn
  as <- getInts
  bs <- getInts
  let ps = permutations bs
      f a b = if a > b then 1 else 0
      g x = if x > n `div` 2 then 1 else 0
      s = sum $ map (g . sum . zipWith f as) ps
      p = fromIntegral s / fromIntegral (product [1..n])
   in print p