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