{-# LANGUAGE BangPatterns, MultiWayIf #-}

import Control.Applicative
import Control.Exception (assert)
import Control.Monad
import Data.List
import Debug.Trace (traceShow, traceShowM)

import Text.Printf (printf)

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

main :: IO ()
main = do
    [n, m, d] <- fmap read . words <$> getLine
    printf "%.7f\n" $ solve n m d