N = int(input())
A = list(map(int, input().split()))
cnt = 0
for i in range(N-2):
    L = set([A[i], A[i+1], A[i+2]])
    if len(L)<3:
        continue
    if A[i+1]==min(L) or A[i+1]==max(L):
        cnt+=1
print(cnt)