N = int(input())
A = list(map(int, input().split()))

count = 0
for a in range(N-2):
    if A[a] > A[a+1] and A[a] < A[a+2]:
        count += 1
    elif A[a] < A[a+1] and A[a] > A[a+2]:
        count += 1
    elif A[a+2] > A[a] and A[a+2] < A[a+1]:
        count += 1
    elif A[a+2] < A[a] and A[a+2] > A[a+1]:
        count += 1

print(count)