object Main extends App {
  val n = io.StdIn.readLine()
  val p = io.StdIn.readLine().split(" ").map(_.toInt)
  val pSize = p.size
  val maxIndex = pSize - 1
  var startIndex = 0
  var result = 0
  while (startIndex + 2 <= maxIndex) {
    val i1 = p(startIndex)
    val i2 = p(startIndex + 1)
    val i3 = p(startIndex + 2)
    if ((i1 <= i2) && (i2 < i3)) result += 1
    else if ((i3 <= i2) && (i2 < i1)) result += 1
    startIndex += 1
  }
  println(result)
}