let read () = Scanf.scanf "%d " (fun i -> i)

let check x y z =
  if x = y || y = z || z = x then false
  else if x < y && y < z then false
  else if x > y && y > z then false
  else true

let () =
  let n = read () in
  let a = Array.init n (fun _ -> read ()) in
  let rec doit i cnt =
    if i = n then cnt
    else doit (i + 1) (cnt + if check a.(i-2) a.(i-1) a.(i) then 1 else 0) in
  doit 2 0 |> Printf.printf "%d\n"