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

nn = n - 2

ans = 0
for i in range(nn):
    if (a[i] > a[i + 1] and a[i + 2] > a[i + 1] and a[i] != a[i + 2]):
        ans += 1
    elif(a[i] < a[i + 1] and a[i + 2] < a[i + 1] and a[i] != a[i + 2]):
        ans += 1
print(ans)