module Main where
 
main :: IO ()
main = getIntList >>= print . solve
 
solve :: [Int] -> Int
solve ab = length $ filter p [a .. b]
  where
  (a:b:_) = ab
  p i = (a + b + i) `mod` 3 == 0
 
getIntList :: IO [Int]
getIntList = map read . words <$> getLine