import math._

object Main {
  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val n = sc.nextInt
    val xs = (1 to n) map { e => sc.nextInt }

    var ans = 0
    for (i <- 1 to xs.length-2) {
      if ((xs(i-1) < xs(i) && xs(i) > xs(i+1)) ||
          (xs(i-1) > xs(i) && xs(i) < xs(i+1))) {
        if (xs(i-1) != xs(i+1)) {
          ans += 1
        }
      }
    }
    println(ans)
  }
}