N = int(input())
A = [int(i) for i in input().split()]
#s = [[int(j) for j in input().split()] for i in range(N)]

res = 0
for i in range(N-2):
    pine = A[i:i+3]
    pine.sort()
    if pine[1]!=A[i+1] and len(set(pine))==3:
        res+=1
print(res)