// No.116 門松列(1)
package main

import (
	"fmt"
	"sort"
)

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

	var a = make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&a[i])
	}

	ans := 0
	for j := 2; j < n; j++ {
		w := []int{a[j-2], a[j-1], a[j]}
		sort.Ints(w)
		if a[j-2] == w[1] || a[j] == w[1] {
			if w[0] != w[1] && w[1] != w[2] && w[2] != w[0] {
				ans++
			}
		}
	}
	fmt.Println(ans)
}