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