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

res = 0

for i in range(n - 2):
    b = a[i:i+3]
    bs = sorted(b)
    if (bs[1] == b[0] or bs[1] == b[2]) and bs[0] != bs[1] and bs[1] != bs[2]:
        res += 1

print(res)