N = int(input())
lsA = list(map(int,input().split()))
cnt = 0
for i in range(N-2):
    if lsA[i] < lsA[i+1] and lsA[i+1] > lsA[i+2] and lsA[i] != lsA[i+2]:
        cnt += 1
    elif lsA[i] > lsA[i+1] and lsA[i+1] < lsA[i+2] and lsA[i] != lsA[i+2]:
        cnt += 1
print(cnt)