func main()
	var n: int :: cui@inputInt()
	var a: []int :: #[n]int
	for i(0, n - 1)
		do a[i] :: cui@inputInt()
	end for
	
	var ans: int
	for i(0, n - 3)
		var x: int :: a[i]
		var y: int :: a[i + 1]
		var z: int :: a[i + 2]
		do ans :+ (x <> z & (y - x) * (y - z) > 0) ?(1, 0)
	end for
	do cui@print("\{ans}\n")
end func