from sys import stdin,stdout

N = int(stdin.readline().replace('\n',''))
x = stdin.readline().replace('\n','').split(' ')

res = 0
for i in range(3, N+1):
    x1,x2,x3 = int(x[i-3]), int(x[i-2]), int(x[i-1])
    if x1==x2 or x2==x3 or x1==x3: continue
    if (x2>x1 and x2>x3) or (x2<x1 and x2<x3): res+=1

stdout.writelines("%d\n" % res)