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

l2 = []
for i in range(n-2):
    l2.append([l[i], l[i+1], l[i+2]])

x = 0
for i in l2:
    if i[0] == i[1] or i[0] == i[2] or i[1] == i[2]:
        pass
    else:
        if min(i) == i[1] or max(i) == i[1]:
            x += 1

print(x)