import Control.Applicative

main :: IO ()
main = solve <$> map read <$> words <$> getLine >>= print

solve :: [Int] -> Int
solve [n, k] | k == 1 = sum $ map (\x -> comb (n-1) x - 1) [0 .. n-1]
             | otherwise = 2 ^ (n-k)

comb n m | m == 0 = 1
         | n < 2 * m = comb n (n-m)
         | otherwise = comb (n-1) (m-1) * n `div` m