let ans, n = ref 0, Scanf.scanf " %d" (+) 0
let ps = Array.init n @@ fun _ -> Scanf.scanf " %d" (+) 0
let _ = for i = 1 to n - 2 do if ps.(i - 1) < ps.(i) && ps.(i) < ps.(i + 1) || ps.(i - 1) > ps.(i) && ps.(i) > ps.(i + 1) then incr ans done; Printf.printf "%d\n" !ans