import Control.Applicative ((<$>))
import Data.Int (Int64)

binom :: Integral a => a -> a -> a
binom n k = product [n, n - 1 .. n - k + 1] `div` product [1 .. k]

solve :: Integral a => a -> a -> a
solve _ 1 = 0
solve n k = sum $ fmap (binom $ n - k) [0 .. n - k]

main :: IO ()
main = do
    [n, k] <- fmap read . words <$> getLine :: IO [Int64]
    print $ solve n k