package main

import "fmt"

func main() {
	var (
		n int
		a []int
	)
	fmt.Scan(&n)

	for range make([]struct{}, n) {
		var ai int
		fmt.Scan(&ai)
		a = append(a, ai)
	}

	ans := 0
	for i := range make([]struct{}, n-2) {
		x := a[i]
		y := a[i+1]
		z := a[i+2]

		if x != z && (y > x && y > z) || (y < x && y < z) {
			ans++
		}
	}
	fmt.Println(ans)
}