Fast.hs

{-# LANGUAGE BangPatterns #-}
import System.Environment (getArgs)

add :: Int -> Int -> Int
add start stop = go (start `mod` 3) (start `mod` 5) 0 start
        where
        go !a !b !acc !i 
                | i == stop = acc
                | otherwise = go a' b' acc' (i+1)
                where
                a' = if a == 2 then 0 else a + 1
                b' = if b == 4 then 0 else b + 1
                acc' = if a == 0 || b == 0 then acc + i else acc

main :: IO ()
main = do
        [start, stop] <- map read <$> getArgs
        print (add start stop)