import           Data.Maybe (fromJust)
import qualified Data.ByteString.Char8 as BSC8

main :: IO ()
main = do
  BSC8.getLine
  xs <- map (fst . fromJust . BSC8.readInt) . BSC8.words <$> BSC8.getLine
  print $ countKadomatsu xs

countKadomatsu :: [Int] -> Int
countKadomatsu = para phi 0
  where
    phi x ([],        st) = st
    phi x ([y],       st) = st
    phi x ([y, z],    st) = st
    phi x ((p:q:r:_), st) = if p /= r && ((p > q && q < r) || (p < q && q > r))
                                then st + 1
                                else st

para :: (a -> ([a], b) -> b) -> b -> [a] -> b
para phi b = step
  where
    step [] = b
    step (x:xs) = phi x (xs, step xs)