package main

import (
	"fmt"
)

func main() {
	var ans int
	var N int
	fmt.Scan(&N)
	foo := make([]int, 100)
	for i:=0;i<N;i++{
		fmt.Scan(&foo[i])
		if i>=2{
			bar := foo[i-2:i+1]
//			fmt.Println(bar)
			if bar[0] < bar[1] && bar[1] > bar[2] && bar[0] != bar[2]{
				ans += 1
			}
			if bar[0] > bar[1] && bar[1] < bar[2] && bar[0] != bar[2]{
				ans += 1
			}
		}

	}
	fmt.Println(ans)
}