import Text.Printf

main :: IO()
main = do
  [a, b, _, k] <- map read . words <$> getLine
  printf "%s" $ slv a b k

slv :: Integer -> Integer -> Integer -> String
slv a b k | abs val > 10^18 = "Unfair"
          | otherwise = show val
  where val | even k = a - b
            | otherwise = b - a