func main()
	var N: int :: cui@input().toInt(&)
	var ss: [][]char :: cui@input().split(" ")
	
	var A: []int :: #[N]int
	for i(0, N - 1)
		do A[i] :: ss[i].toInt(&)
	end for
	
	var ans: int
	for i(0, N - 3)
		var a: int :: A[i]
		var b: int :: A[i + 1]
		var c: int :: A[i + 2]
		do ans :+ (a <> c & (b - a) * (b - c) > 0) ?(1, 0)
	end for
	do cui@print("\{ans}\n")
end func