N = int(input())
f = input()
s_f = f.split()
length = [int(v) for v in s_f]

i = 0
for v in range(0, N - 2):
    a = {length[v],length[v + 1],length[v + 2]}
    if len(a) == 3:
        if length[v] < length[v + 1] and length[v + 1] > length[v + 2]:
            i += 1
        elif length[v] > length[v + 1] and length[v + 1] <length[v + 2]:
            i += 1

print(i)