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

def kadomatu(a, b, c):
    if b < a < c:
        return True
    if c < a < b:
        return True
    if a < c < b:
        return True
    if b < c < a:
        return True
    return False

ans = 0
for i in range(N-2):
    ans += kadomatu(*A[i:i+3])
print(ans)