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 + 2] != a[i] and (a[i + 1] - a[i]) * (a[i + 1] - a[i + 2]) > 0:
        ans += 1
print(ans)