n = int(input())
arr = list(map(int, input().split()))

ans = 0

for i in range(1, n - 1):
    if arr[i - 1] != arr[i] and arr[i - 1] != arr[i + 1] and arr[i] != arr[i + 1]:
        if(arr[i - 1] < arr[i] < arr[i + 1] or arr[i - 1] > arr[i] > arr[i + 1]):
            continue
        ans += 1

print(ans)