{-# LANGUAGE TupleSections     #-}

import Control.Applicative
import Control.Monad (guard)
import Data.List (unfoldr)

g = sum 
          . filter even
          . unfoldr ( \x -> fmap(x,) $
                   x`div`2 <$ guard(even x)
               <|> x*3 + 1 <$ guard( x/=1 )
             )
             
main = print $ g 201