let _ = stdin.ReadLine()
let l = stdin.ReadLine().Split(' ') |> Seq.map int |> Seq.toList
let rec check a b c =
    (a <> b && b <> c && a <> c)
    && ((a > b && b < c) || (a < b && b > c))
let rec f cnt = function
    | a::b::c::xs ->
        let cnt = if check a b c then cnt + 1 else cnt
        f cnt ([b;c]@xs)
    | _ -> cnt
f 0 l
|> printfn "%d"