b=int(input()) a=input().split() c=0 for i in range(1,b-1): if a[i-1]>a[i] and a[i]<=a[i+1] or a[i-1]>=a[i] and a[i]<a[i+1]: c+=1 if a[i-1]==a[i]: c-=1 print(c)