package main

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

func main() {
	var n, count int
	nums := make([]int, 0, 100)

	fmt.Scanln(&n)
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Scan()

	for _, v := range strings.Split(scanner.Text(), " ") {
		i, _ := strconv.Atoi(v)
		nums = append(nums, i)
	}

	for i := 0; i < n-2; i++ {
		col := nums[i : i+3]
		if (col[1] < col[0] && col[0] < col[2]) || (col[2] < col[0] && col[0] < col[1]) {
			count++
			continue
		}

		if (col[0] < col[2] && col[2] < col[1]) || (col[1] < col[2] && col[2] < col[0]) {
			count++
		}
	}
	fmt.Println(count)
}