N = int(input())
A = [int(i) for i in input().split()]
B = 0
for i in range(N-2):
    if A[i] == A[i+1] or A[i+1] == A[i+2] or A[i] == A[i+2]:
        B = B
    elif A[i]<A[i+1] and A[i+2]<A[i+1]:
        B += 1
    elif A[i]>A[i+1] and A[i+2]>A[i+1]:
        B += 1
print(B)