{- _prime :: Int -> Int -> Int -> Bool
_prime n i max	| i > max = True
		| rem n i == 0 = False
		| otherwise = _prime n (i + 1) max

prime :: Int -> Bool
prime n = _prime n 2 (ceiling (sqrt (fromIntegral n)))

-- Cannot handle prime nubmers
_first_factor :: Int -> Int -> Int -> Int
_first_factor n i max 	| rem n i == 0 = i
			| otherwise = _first_factor n (i+1) max

first_factor n = _first_factor n 2 (ceiling (sqrt (fromIntegral n)))

factor :: Int -> [Int]
factor n	| prime n = [n]
		| otherwise = (factor x) ++ (factor y) where 
			x = first_factor n
			y = div n x
-}

_factor n divisor 	| divisor > (ceiling (sqrt (fromIntegral n))) = [n]
			| rem n divisor == 0 = (_factor (div n divisor) 2) ++ (_factor divisor 2)
			| otherwise = _factor n (divisor + 1)

factor n = _factor n 2

main = print (maximum (factor 600851475143))