n = int(input())
a = list(map(int, input().split()))
num = 0
for i in range(1, n - 1):
    if a[i - 1] < a[i] and a[i] > a[i + 1] and a[i - 1] != a[i + 1]:
        num += 1
    elif a[i - 1] > a[i] and a[i] < a[i + 1] and a[i - 1] != a[i + 1]:
        num += 1

print(num)