n = int(raw_input())
l = map(int, raw_input().split())
c = 0
for i in range(n-2):
    x = l[i]
    y = l[i+1]
    z = l[i+2]
    if x != y and y != z and z != x and (min((x,y,z)) == y or max((x,y,z)) == y):
        c += 1
print c