n = int(input())
a = [int(i) for i in input().split()]

count = 0

for i in range(n-2):
    x = a[i]
    y = a[i+1]
    z = a[i+2]

    if x != y and y != z and z != x:
        if y == max(x,y,z) or y == min(x,y,z):
            count += 1

print(count)