n = int(input())
l = list(map(int,input().split()))
s = 0
for i in range(n-2):
    if l[0] != l[2] and ((l[0] < l[1] and l[1] > l[2]) or (l[0] > l[1] and l[1] < l[2])):
        s += 1
    l.pop(0)
print(s)