n = int(input())
m = list(map(int,input().split()))
c = 0
for i in range(0,n-2):
    if m[i]!=m[i+2]:
        if m[i]<m[i+1]>m[i+2] or m[i]>m[i+1]<m[i+2]:
            c+=1
print(c)