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

ans = 0
for i in range(N-2):
    if A[i] != A[i+1] and A[i+1] != A[i+2] and A[i] != A[i+2]:
        if A[i] > A[i+1] < A[i+2]:
            ans += 1
        elif A[i] < A[i+1] > A[i+2]:
            ans += 1
print(ans)