B.hs

line :: Proc String -- same as getLine

solveB :: Proc String
solveB = do
	[nS,vS,xS] <- words <$> line
	let
		n = read nS :: Int
		[v,x] = map read [vS,xS] :: [Double]
	ss <- map (map read . words) <$> replicateM n line
	let
		minS = map (\xs -> (sum $ map fst xs, snd $ head xs)) $ groupBy ((==)`on`snd) $ map (\[r,c] -> (r,c)) $ sortBy (compare`on`(!!1)) ss
		maxS = reverse minS
		maxT = (*2) $ v / (minimum $ map fst minS) + 0.1
		temp :: [(Double,Double)] -> Double -> Maybe Double
		temp xs t = toMaybe $ foldl f (Left (0,0)) xs where
			f (Right x) _ = Right x
			f (Left (p,c)) (u,y)
				| p + t*u < v = Left (p+t*u, c+u*y*t)
				| otherwise = Right $ c/v+(1-p/v)*y
			toMaybe (Left (p,c)) = Nothing
			toMaybe (Right x) = Just x
		check :: Double -> Bool
		check t = let
				nT = temp minS t
				xT = temp maxS t
			in case (nT,xT) of
				(Just p, Just q) -> p <= x && x <= q
				_ -> False
		bSearch :: (Double,Double) -> Double
		bSearch (l,r)
			| abs (r-l) < 1e-8 = (l+r)/2
			| otherwise = let t = (l+r)/2 in case check t of
					True  -> bSearch (l,t)
					False -> bSearch (t,r)
	return $ case check maxT of
		True  -> show $ bSearch (0,maxT)
		False -> "IMPOSSIBLE"