m = int(input())
n = list(map(int, input().split()))
count = 0
for i in range(m - 2):
    a = list(map(int, n[0 + i:3 + i]))
    if a[0] < a[1] and a[1] > a[2] and a[0] != a[2]:
        count += 1
    elif a[0] > a[1] and a[1] < a[2] and a[0] != a[2]:
        count += 1
print(count)