ans = 0

N = int(input())

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

for i in range(N - 2):
    
    if A[i] != A[i + 2] and ((A[i] < A[i + 1] and A[i + 2] < A[i + 1]) or (A[i] > A[i + 1] and A[i + 2] > A[i + 1])):
        
        ans += 1
    
print(ans)