n = int(input())
a = list(map(int, input().split()))
count = 0

for i in range(n - 2):
    x, y, z = a[i], a[i+1], a[i+2]
    if len({x, y, z}) != 3:
        continue
    sorted_three = sorted([x, y, z], reverse=True)
    mid = sorted_three[1]
    if mid == x or mid == z:
        count += 1

print(count)