N = int(input())
A = list(map(int,input().split()))
ans = 0
for i in range(N-2):
    if A[i] != A[i+2] and (A[i+1] < min(A[i],A[i+2]) or A[i+1] > max(A[i],A[i+2])):
        ans += 1
print(ans)