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 x == y or y == z or x == z:
        continue
    if y == max(x, y, z) or y == min(x, y, z):
        count += 1
print(count)