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

x = 0
for i in range(n-2):
    b = a[i+2]
    c = a[i+1]
    d = a[i]
    if b != c and c != d and b != d and (b < c > d or b > c < d):
        x += 1

print(x)