import Control.Applicative
import Text.Printf

main = do
  [h, a, d] <- map read . words <$> getLine
  let
    max_k = (h+a-1) `div` a
    f :: Int -> Double
    f h'
      | h' <= 0 = 0
      | otherwise = 3 / 2 * (fromIntegral l)
      where
        l = (h'+d-1) `div` d
    mn = minimum $ map (\t -> fromIntegral t + f (h-a*t)) [0..max_k]
   in printf "%.3f\n" mn