import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	A = getlist()
	ans = 0
	for i in range(N - 2):
		if A[i] > A[i + 1] and A[i + 1] < A[i + 2] and A[i] != A[i + 2]:
			ans += 1
		elif A[i] < A[i + 1] and A[i + 1] > A[i + 2] and A[i] != A[i + 2]:
			ans += 1

	print(ans)


if __name__ == '__main__':
	main()