iogetfloats :: IO [Double]
iogetfloats = do
  s <- getLine
  return $ map read $ words s

solve :: Double -> Double -> Double -> Double
solve n m d
	| d == 0 = (m - 1) / n
	| otherwise = 2 * (n - d) * (m - 1) / (n * n)

main :: IO ()
main = do 
	[n, m, d] <- iogetfloats
	print (solve n m d)