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

ans = 0
for i in range(n-2):
    if A[i] == A[i+1] or A[i+1] == A[i+2] or A[i+2] == A[i]:
        continue
    if A[i] < A[i+1] and A[i+1] > A[i+2]:
        ans += 1
    if A[i] > A[i+1] and A[i+1] < A[i+2]:
        ans += 1
print(ans)