N = int(input())
A = list(map(int,input().split()))

ans = 0
for i in range(len(A)-2):
    l = A[i]
    m = A[i+1]
    r = A[i+2]
    if (m > l and m > r and l != r) or (m < l and m < r and l != r):
        ans += 1
print(ans)