#少し修正
N = int(input())
k = list(map(int, input().split()))
c = 0
for i,v in enumerate(k[1:-1],1):
    if len(set([k[i-1], v ,k[i+1]])) == 3:
        if k[i-1] > v < k[i+1] or k[i-1] < v > k[i+1]:
            c += 1
print(c)