package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	N, _ := strconv.Atoi(sc.Text())
	sc.Scan()
	strs := strings.Split(sc.Text(), " ")
	nums := make([]int, N)
	for i, str := range strs {
		nums[i], _ = strconv.Atoi(str)
	}
	c := 0
	for i := 0; i < N-2; i++ {
		if nums[i] < nums[i+1] && nums[i+1] > nums[i+2] || nums[i] > nums[i+1] && nums[i+1] < nums[i+2] {
			if nums[i] != nums[i+2] {
				c++
			}
		}
	}
	fmt.Println(c)
}