N = int(input())
A = list(map(int, input().split()))
c = 0
for n in range(1,N-1):
    if (A[n-1] > A[n] < A[n+1] or A[n-1] < A[n] > A[n+1]) and A[n-1] != A[n+1]:
            c += 1

print(c)