main = do
    [n,m,d] <- map read . words <$> getLine :: IO [Int]
    let p2 = if d == 0 then 0 else length [d+1..n-d]
        p1 = n - p2
        ans = fromIntegral ((p1 + 2 * p2) * (m-1)) / fromIntegral n / fromIntegral n
    print ans