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

def kado(a, b, c):
    if a == b or a == c or b == c:
        return False
    if max(a, b, c) == b or min(a, b, c) == b:
        return True
    return False
    
ans = 0
for i in range(n - 2):
    if kado(a[i], a[i + 1], a[i + 2]):
        ans += 1
print(ans)