n = int(input())
s = list(map(int, input().split()))

cnt = 0
for i in range(n-2):
    a = s[i]
    b = s[i+1]
    c = s[i+2]
    if a < b and b > c and a != c:
        cnt += 1
    elif a > b and b < c and a != c:
        cnt += 1

print(cnt)