import Control.Applicative
import Control.Monad
import Data.List
import Text.Printf

solve :: [Int] -> Double    
solve [n, m, d] = fromIntegral (m - 1) * avg
  where
    avg = (if d==0 then 1.0 else 2.0) * (1.0 - fromIntegral d / fromIntegral n) / fromIntegral n

main = map read . words <$> getLine >>= putStrLn . printf "%.15f" . solve