N = int(input())
A = [int(i) for i in input().split()]
res = 0
for i in range(N-2):
    l = A[i:i+3]
    l_s = sorted(l)
    if (l[0] == l_s[1] or l[2] == l_s[1]) and l[0] != l[1] != l[2]:
        res += 1
print(res)