N = int(input())
A = list(map(int, input().split()))
ans = 0
for i in range(1, N - 1):
    l, c, r = A[i - 1], A[i], A[i + 1]
    if l != c and c != r and r != l:
        if not (l < c < r) and not (l > c > r):
            ans += 1
print(ans)