def is_kadomatsu(a, b, c):
    if a == b or b == c or c == a:
        return False
    if a < b > c or a > b < c:
        return True
    else:
        return False

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

ans = 0
for i in range(n - 2):
    if is_kadomatsu(A[i], A[i + 1], A[i + 2]):
        ans += 1
print(ans)