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],k[i],k[i+1]])) == 3:
        if k[i-1] > k[i] < k[i+1] or k[i-1] < k[i] > k[i+1]:
            c += 1
print(c)