package main

import (
	"fmt"
	"io"
	"os"
)

func solve(in io.Reader, out, err io.Writer) {
	num, total := 0, 0
	fmt.Fscan(in, &num)
	bamboo := make([]int, num)

	for i := range bamboo {
		fmt.Fscan(in, &bamboo[i])
		if i >= 2 {
			a, b, c := bamboo[i-2], bamboo[i-1], bamboo[i]
			if a != c && ((a > b && c > b) || (b > a && b > c)) {
				total++
			}
		}
	}

	fmt.Fprintln(out, total)
}

func main() {
	solve(os.Stdin, os.Stdout, os.Stderr)
}