N = input()
A = map(int, raw_input().split())
res = 0
for i in range(N - 2):
    a = A[i]
    b = A[i + 1]
    c = A[i + 2]
    if (a == b or b == c):
        continue
    if (b < a or b > c):
        res += 1
print res